View Filing Data

Finite-lived intangible assets, Gross Carrying Amount

Goodwill

$ 4,523

1,757

1,771

Finite-lived intangible assets, Accumulated Amortization

1,313

1,234

Indefinite-lived intangible assets, Gross Carrying Amount

$ 3,095

3,095

Frequent Flyer Database

Finite-lived intangible assets, Gross Carrying Amount

^[1],[2]

22 years

^[1]

$ 1,177

1,177

Finite-lived intangible assets, Accumulated Amortization

^[1]

$ 832

771

Hubs

Finite-lived intangible assets, Gross Carrying Amount

^[2]

20 years

$ 145

145

Finite-lived intangible assets, Accumulated Amortization

$ 89

Contracts

Finite-lived intangible assets, Gross Carrying Amount

^[2]

13 years

$ 121

135

Finite-lived intangible assets, Accumulated Amortization

$ 103

Patents And Trade Names

Finite-lived intangible assets, Gross Carrying Amount

^[2]

3 years

$ 108

108

Finite-lived intangible assets, Accumulated Amortization

$ 108

108

Airport Slots and Gates

Finite-lived intangible assets, Gross Carrying Amount

^[2]

8 years

$ 97

Finite-lived intangible assets, Accumulated Amortization

$ 97

Other Intangible Assets

Finite-lived intangible assets, Gross Carrying Amount

^[2]

25 years

$ 109

109

Finite-lived intangible assets, Accumulated Amortization

Route Authorities

Indefinite-lived intangible assets, Gross Carrying Amount

1,562

Airport Slots And Gates

Indefinite-lived intangible assets, Gross Carrying Amount

536

Tradenames And Logos

Indefinite-lived intangible assets, Gross Carrying Amount

593

Alliances

Indefinite-lived intangible assets, Gross Carrying Amount

$ 404

[1]	The frequent flyer database is amortized based on an accelerated amortization schedule to reflect utilization of the assets. Estimated cash flows correlating to the expected attrition rate of customers in the frequent flyer database is considered in the determination of the amortization schedules.
[2]	Weighted average life expressed in years.

Goodwill and Other Intangible Assets - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Actual and Estimated Amortization Expense [Line Items]
Actual amortization	$ 79	$ 90	$ 105
Projected amortization in 2018	67
Projected amortization in 2019	61
Projected amortization in 2020	55
Projected amortization in 2021	50
Projected amortization in 2022	$ 40

Common Stockholders' Equity and Preferred Securities - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule Of Stockholders Equity [Line Items]
Number of shares repurchased	28,000,000
Repurchases of common stock	$ 1,844	$ 2,607	$ 1,232
Share repurchase program additional amount authorized	3,000
Amount remaining under repurchase programs	$ 3,000
Treasury stock, shares, retired	25,000,000
Treasury stock retired, average cost per share	$ 63
Common stock reserved for future issuance	10,000,000
Junior preferred stock outstanding	2
Junior preferred stock par value per share	$ 0.01
Preferred stock authorized to issue	250,000,000

Computation of Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Computation of Earnings Per Share [Line Items]
Earnings available to common stockholders									$ 2,131	$ 2,263	$ 7,340
Basic weighted-average shares outstanding									302.7	329.9	376.1
Effect of convertible notes											0.3
Effect of employee stock awards									0.9	0.4	0.5
Diluted weighted-average shares outstanding									303.6	330.3	376.9
Earnings per share, basic	$ 1.99	$ 2.12	$ 2.67	$ 0.31	$ 1.26	$ 3.02	$ 1.78	$ 0.88	$ 7.04	$ 6.86	$ 19.52
Earnings per share, diluted	$ 1.99	$ 2.12	$ 2.66	$ 0.31	$ 1.26	$ 3.01	$ 1.78	$ 0.88	$ 7.02	$ 6.85	$ 19.47

Share-Based Compensation Plans - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Equity awards fair value	$ 76.0	$ 80.0	$ 92.0
Outstanding stock option awards	500,000
Outstanding stock option awards, exercisable	100,000
Outstanding weighted average exercise price	$ 51.67
Outstanding exercisable weighted average exercise price	$ 34.74
Outstanding intrinsic value	$ 8.0
Outstanding exercisable intrinsic value	$ 5.0
Outstanding weighted average remaining contractual live	6 years 3 months 19 days
Outstanding exercisable weighted average remaining contractual live	3 years 8 months 12 days
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, other than options, awards granted	1,600,000	1,900,000	1,000,000
Liabilities related to share based payments	$ 38.0
Payment related to share-based liabilities	$ 50.0	$ 69.0	$ 85.0
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, options, awards granted	36,000
Share-Based Compensation, options term	10 years
Percentage of premium of the grant date fair market value		25.00%
Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, Weighted Average Exercise Price	$ 77.56	$ 56.19
Share-based Compensation Arrangements by Share-based Payment Award, Options, Grants in Period, grant date fair value	$ 0.7	$ 2.3
Stock Options \| Exercise Prices Equal to Fair Market Value
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, options, awards granted	36,000.0	100,000	0
Stock Options \| Stock options with exercise prices at a 25% premium of the grant date fair market value
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, options, awards granted		300,000
Time-Vested RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, other than options, awards granted	1,000,000
Share-Based Compensation, vesting period	3 years
Number of days used to compute performance period average closing price of restricted stock units	20 days
Performance-Based RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation, other than options, awards granted	600,000
Share-Based Compensation, vesting period	3 years
Number of days used to compute performance period average closing price of restricted stock units	20 days
Share-Based Compensation, vesting date	Dec. 31, 2019

Schedule of Share-Based Compensation Plan Cost (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost	$ 73	$ 70	$ 58
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost	63	58	52
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost	8	11	$ 6
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Compensation cost	$ 2	$ 1

Schedule of Unearned Compensation and Weighted Average Remaining Period for Outstanding Share-Based Awards (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unearned Compensation	$ 53
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unearned Compensation	$ 46
Weighted- Average Remaining Period (in years)	1 year 10 months 25 days
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unearned Compensation	$ 4
Weighted- Average Remaining Period (in years)	3 years 10 months 25 days
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unearned Compensation	$ 3
Weighted- Average Remaining Period (in years)	1 year 2 months 12 days

Schedule of Restricted Stock Unit and Restricted Stock Activity (Detail) - $ / shares shares in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding at beginning of year	0.5	0.3	0.7
Granted, Stock		0.4	0.2
Vested, Stock	(0.2)	(0.1)	(0.4)
Forfeited, Stock		(0.1)	(0.2)
Outstanding at end of year, Stock	0.3	0.5	0.3
Weighted-Average Grant Price at beginning of year	$ 52.00	$ 48.68	$ 32.55
Granted, Weighted-Average Grant Price		50.63	66.53
Vested, Weighted-Average Grant Price	51.60	41.47	31.14
Forfeited, Weighted-Average Grant Price	51.88	53.42	46.23
Outstanding at end of year, Weighted-Average Grant Price	$ 52.30	$ 52.00	$ 48.68
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding at beginning of year	2.9	2.6	3.8
Granted, Stock	1.6	1.9	1.0
Vested, Stock	(1.0)	(1.4)	(1.6)
Forfeited, Stock	(0.3)	(0.2)	(0.6)
Outstanding at end of year, Stock	3.2	2.9	2.6

Components of Accumulated Other Comprehensive Income, Net of Tax (Detail) - USD ($)
$ in Millions

12 Months Ended

Dec. 31, 2017

Dec. 31, 2016

Dec. 31, 2015

Accumulated Other Comprehensive Income (Loss) [Line Items]

Beginning Balance, deferred taxes

$ 26

$ (256)

$ (116)

Other comprehensive income (loss) before reclassifications, deferred taxes

187

Amounts reclassified from accumulated other comprehensive income, deferred taxes

(21)

(228)

Reclassification of stranded tax effects

^[1]

(118)

Net other comprehensive income (loss), deferred taxes

(65)

282

(140)

Ending Balance, deferred taxes

(39)

^[2]

(256)

Beginning Balance

8,659

8,966

2,396

Other comprehensive income (loss) before reclassifications, net of tax

(239)

(334)

(159)

Amounts reclassified from accumulated other comprehensive income, net of tax

336

407

Total other comprehensive income (loss), net

(318)

248

Ending Balance

8,806

8,659

8,966

Accumulated Other Comprehensive Income (Loss)

Accumulated Other Comprehensive Income (Loss) [Line Items]

Beginning Balance

(829)

(831)

(1,079)

Reclassification of stranded tax effects

(118)

Ending Balance

(1,147)

(829)

(831)

Pension and Other Postretirement Liabilities

Accumulated Other Comprehensive Income (Loss) [Line Items]

Beginning Balance, before tax

(854)

(363)

(472)

Other comprehensive income (loss) before reclassifications, before tax

^[3]

(306)

(517)

Amounts reclassified from accumulated other comprehensive income, before tax

Net other comprehensive income (loss), before tax

(248)

(491)

109

Ending Balance, before tax

(1,102)

(854)

(363)

Derivative Contracts | Fuel

Accumulated Other Comprehensive Income (Loss) [Line Items]

Beginning Balance, before tax

(2)

(215)

(499)

Other comprehensive income (loss) before reclassifications, before tax

(4)

(320)

Amounts reclassified from accumulated other comprehensive income, before tax

217

604

Net other comprehensive income (loss), before tax

213

284

Ending Balance, before tax

(2)

(215)

Investments and Other

Accumulated Other Comprehensive Income (Loss) [Line Items]

Beginning Balance, before tax

Other comprehensive income (loss) before reclassifications, before tax

(7)

(5)

Amounts reclassified from accumulated other comprehensive income, before tax

(2)

Net other comprehensive income (loss), before tax

(7)

(2)

(5)

Ending Balance, before tax

$ (6)

$ 1

$ 3

[1]	This amount represents the reclassification from AOCI to RE of the stranded tax effects resulting from the enactment of the Tax Act.
[2]	Deferred tax balance relates mainly to Pension and Other Postretirement Liabilities.
[3]	Prior service credits increased by $0 million, $30 million and $0 million and actuarial losses increased by approximately $306 million, $560 million and $78 million for 2017, 2016 and 2015, respectively.

Details about Accumulated Other Comprehensive Income Components (Detail) - USD ($)
$ in Millions

12 Months Ended

Dec. 31, 2017

Dec. 31, 2016

Dec. 31, 2015

Salaries And Related Costs

Accumulated Other Comprehensive Income (Loss) [Line Items]

Amortization of unrecognized (gains) losses and prior service cost and the effect of curtailments and settlements

^[1]

$ 58

$ 26

$ 31

Miscellaneous, Net

Accumulated Other Comprehensive Income (Loss) [Line Items]

Reclassifications of gains into earnings

(2)

Fuel Oil Contract | Aircraft Fuel

Accumulated Other Comprehensive Income (Loss) [Line Items]

Reclassifications of losses into earnings

^[2]

$ 2

$ 217

$ 604

[1]

This AOCI component is included in the computation of net periodic pension and other postretirement costs (see Note 8 of this report for additional information).

[2]

The last of the Company's fuel hedge derivatives designated for cash flow hedge accounting expired in December 2016. The 2017 amount reclassified from AOCI into fuel expense represents hedge losses on December 2016 settled trades, but for which the associated fuel purchased in December 2016 was not consumed until January 2017. The Company's current strategy is to not enter into transactions to hedge its fuel consumption, although the Company regularly reviews its strategy based on market conditions and other factors.

Components of Accumulated Other Comprehensive Income, Net of Tax (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Increase in prior service credits	$ 0	$ 30	$ 0
Increase in actuarial losses	$ 306	$ 560	$ 78

Accumulated Other Comprehensive Income (Loss) - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2017	Sep. 30, 2015
Accumulated Other Comprehensive Income (Loss) [Line Items]
Valuation allowance	$ 68	$ 63
Income tax adjustment from AOCI	$ 180
Valuation allowance for deferred tax assets
Accumulated Other Comprehensive Income (Loss) [Line Items]
Valuation allowance			$ 465
Pension and Other Postretirement Liabilities \| Valuation allowance for deferred tax assets
Accumulated Other Comprehensive Income (Loss) [Line Items]
Valuation allowance			285
Fuel \| Designated as Hedging Instrument \| Valuation allowance for deferred tax assets
Accumulated Other Comprehensive Income (Loss) [Line Items]
Valuation allowance			$ 180

Income Tax Provision Differed from Amounts Computed at Statutory Federal Income Tax Rate and Significant Components (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Income tax provision at statutory rate	$ 1,050	$ 1,337	$ 1,477
State income taxes, net of federal income tax	29	38	38
Foreign tax rate differential	(43)
Foreign income taxes	3	3	4
Nondeductible employee meals	17	16	15
Impact of Tax Act	(192)
Income tax adjustment from AOCI		180
State rate change	12	(12)
Valuation allowance	(16)	20	(4,662)
Other, net	8	(26)	7
Income tax expense (benefit)	868	1,556	(3,121)
Current	(77)	(92)	56
Deferred	945	1,648	(3,177)
Income tax expense (benefit)	868	1,556	(3,121)
United Airlines, Inc.
Income Taxes [Line Items]
Income tax provision at statutory rate	1,051	1,338	1,477
State income taxes, net of federal income tax	29	38	38
Foreign tax rate differential	(43)
Foreign income taxes	3	3	4
Nondeductible employee meals	17	16	15
Impact of Tax Act	(209)
Income tax adjustment from AOCI		180
State rate change	12	(12)
Valuation allowance	(16)	20	(4,621)
Other, net	8	(25)	7
Income tax expense (benefit)	852	1,558	(3,080)
Current	(77)	(92)	56
Deferred	929	1,650	(3,136)
Income tax expense (benefit)	$ 852	$ 1,558	$ (3,080)

Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Federal statutory income tax rate				35.00%		35.00%
Deferred income tax expense				$ 945	$ 1,648	$ (3,177)
Income tax benefit				192
Provisional one-time transition tax on previously deferred foreign earnings				19
Undistributed cumulative earnings of foreign subsidiaries contributing to one-time transition tax	$ 122			122
Income tax adjustments	(192)	$ 180		(192)	180	(3,130)
Valuation allowance	63	68		63	68
Unrecognized tax benefits	21	$ 74		21	74	24
Unrecognized tax benefits that would impact effective tax rate if recognized	21			$ 21
Scenario Forecast
Income Taxes [Line Items]
Federal statutory income tax rate			21.00%
Minimum
Income Taxes [Line Items]
State tax benefits of the NOL carry forwards expiration period, years				5 years
Maximum
Income Taxes [Line Items]
State tax benefits of the NOL carry forwards expiration period, years				20 years
2026
Income Taxes [Line Items]
Net operating loss expiration amount	200			$ 200
2029
Income Taxes [Line Items]
Net operating loss expiration amount	500			500
Thereafter
Income Taxes [Line Items]
Net operating loss expiration amount	1,700			1,700
Fuel \| Designated as Hedging Instrument
Income Taxes [Line Items]
Deferred income tax expense					$ 180	$ 180
Federal
Income Taxes [Line Items]
NOL carry forwards	2,400			2,400
State
Income Taxes [Line Items]
NOL carry forwards	49			49
NOL carry forwards, net of valuation allowance	52			52
Valuation allowance	$ 63			$ 63

Components of Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Deferred income tax asset (liability):
Federal and state net operating loss ("NOL") carryforwards	$ 601	$ 1,613
Deferred revenue	1,069	2,096
Employee benefits, including pension, postretirement and medical	1,051	1,662
Alternative minimum tax credit carryforwards		116
Other	351	523
Less: Valuation allowance	(63)	(68)
Total deferred tax assets	3,009	5,942
Depreciation	(2,431)	(3,961)
Intangibles	(803)	(1,326)
Total deferred tax liabilities	(3,234)	(5,287)
Net deferred tax asset		655
Net deferred tax liabilities	(225)
United Airlines, Inc.
Deferred income tax asset (liability):
Federal and state net operating loss ("NOL") carryforwards	574	1,571
Deferred revenue	1,069	2,096
Employee benefits, including pension, postretirement and medical	1,051	1,662
Alternative minimum tax credit carryforwards		116
Other	351	522
Less: Valuation allowance	(63)	(68)
Total deferred tax assets	2,982	5,899
Depreciation	(2,431)	(3,961)
Intangibles	(803)	(1,326)
Total deferred tax liabilities	(3,234)	(5,287)
Net deferred tax asset		$ 612
Net deferred tax liabilities	$ (252)

Reconciliation of Change in Projected Benefit Obligation and Plan Assets (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation:	$ 4,739	$ 4,158
Benefit obligation at beginning of year	5,253	4,473
Service cost	195	112	$ 124
Interest cost	220	200	200
Actuarial (gain) loss	525	738
Gross benefits paid and settlements	(366)	(243)
Other	25	(27)
Benefit obligation at end of year	5,852	5,253	4,473
Fair value of plan assets at beginning of year	3,355	2,975
Actual return on plan assets	510	230
Employer contributions	419	421
Gross benefits paid and settlements	(366)	(243)
Other	14	(28)
Fair value of plan assets at end of year	3,932	3,355	2,975
Funded status-Net amount recognized	(1,920)	(1,898)
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of year	1,687	2,002
Service cost	13	19	21
Interest cost	66	86	82
Plan participants' contributions	68	69
Benefits paid	(178)	(191)
Actuarial (gain) loss	40	(165)
Plan amendments		(138)
Other	14	5
Benefit obligation at end of year	1,710	1,687	2,002
Fair value of plan assets at beginning of year	55	56
Actual return on plan assets	1	2
Employer contributions	108	119
Plan participants' contributions	68	69
Benefits paid	(178)	(191)
Fair value of plan assets at end of year	54	55	$ 56
Funded status-Net amount recognized	$ (1,656)	$ (1,632)

Amounts Recognized in Consolidated Balance Sheet and Accumulated Other Comprehensive Income (Loss) (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Amounts recognized in the consolidated balance sheets consist of:
Noncurrent asset	$ 9	$ 2
Current liability	(8)	(8)
Noncurrent liability	(1,921)	(1,892)
Total liability	(1,920)	(1,898)
Amounts recognized in accumulated other comprehensive income (loss) consist of:
Net actuarial gain (loss)	(1,610)	(1,482)
Prior service credit (loss)	(1)	(1)
Total accumulated other comprehensive income (loss)	(1,611)	(1,483)
Other Postretirement Benefits
Amounts recognized in the consolidated balance sheets consist of:
Current liability	(54)	(51)
Noncurrent liability	(1,602)	(1,581)
Total liability	(1,656)	(1,632)
Amounts recognized in accumulated other comprehensive income (loss) consist of:
Net actuarial gain (loss)	301	384
Prior service credit (loss)	208	245
Total accumulated other comprehensive income (loss)	$ 509	$ 629

Accumulated Benefit Obligation and Projected Benefit Obligation in Excess of Plan Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 5,637	$ 5,025
Accumulated benefit obligation	4,567	3,985
Fair value of plan assets	$ 3,709	$ 3,164

Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 195	$ 112	$ 124
Interest cost	220	200	200
Expected return on plan assets	(243)	(216)	(194)
Amortization of unrecognized actuarial (gain) loss	128	76	85
Other	5	5	4
Net periodic benefit cost (credit)	305	177	219
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	13	19	21
Interest cost	66	86	82
Expected return on plan assets	(2)	(2)	(2)
Curtailment gain		(107)
Amortization of unrecognized actuarial (gain) loss	(33)	(19)	(22)
Amortization of prior service credits	(37)	(31)	(32)
Net periodic benefit cost (credit)	$ 7	$ (54)	$ 47

Amortized Accumulated Other Comprehensive Income (Loss) Into Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Actuarial (gain) loss	$ 132
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Actuarial (gain) loss		$ (32)
Prior service (credit) cost		$ (37)

Assumptions Used for Benefit Plans (Detail)	12 Months Ended
Assumptions Used for Benefit Plans (Detail)	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Assumptions used to determine benefit obligations
Discount rate	3.65%	4.18%
Rate of compensation increase	3.89%	3.54%
Assumptions used to determine net expense
Discount rate	4.19%	4.58%
Expected return on plan assets	7.02%	7.04%
Rate of compensation increase	3.54%	3.53%
Other Postretirement Benefits
Assumptions used to determine benefit obligations
Discount rate	3.63%	4.07%
Assumptions used to determine net expense
Discount rate	4.07%	4.49%
Expected return on plan assets	3.00%	3.00%
Health care cost trend rate assumed for next year	6.25%	6.50%
Rate to which the cost trend rate is assumed to decline (ultimate trend rate in 2023)	5.00%	5.00%

Allocation of Plan Assets (Detail) - United Airlines, Inc.	12 Months Ended
	Dec. 31, 2017
Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Expected Long-Term Rate of Return	9.50%
Equity Securities \| Minimum
Defined Benefit Plan Disclosure [Line Items]
Percent of Total	27.00%
Equity Securities \| Maximum
Defined Benefit Plan Disclosure [Line Items]
Percent of Total	42.00%
Fixed-Income Securities
Defined Benefit Plan Disclosure [Line Items]
Expected Long-Term Rate of Return	5.50%
Fixed-Income Securities \| Minimum
Defined Benefit Plan Disclosure [Line Items]
Percent of Total	30.00%
Fixed-Income Securities \| Maximum
Defined Benefit Plan Disclosure [Line Items]
Percent of Total	40.00%
Alternatives
Defined Benefit Plan Disclosure [Line Items]
Expected Long-Term Rate of Return	7.30%
Alternatives \| Minimum
Defined Benefit Plan Disclosure [Line Items]
Percent of Total	10.00%
Alternatives \| Maximum
Defined Benefit Plan Disclosure [Line Items]
Percent of Total	25.00%
Other Investments
Defined Benefit Plan Disclosure [Line Items]
Expected Long-Term Rate of Return	7.30%
Other Investments \| Minimum
Defined Benefit Plan Disclosure [Line Items]
Percent of Total	0.00%
Other Investments \| Maximum
Defined Benefit Plan Disclosure [Line Items]
Percent of Total	10.00%

Pension and Other Postretirement Plans (Defined Pension and Other Postretirement Plans) - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Effect on post retirement benefit obligation, 1% Decrease		$ 185
Effect on benefit expense, 1% Decrease		8
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Employer contributions		$ 108	$ 119
Other Postretirement Benefits \| Deposit Administration Fund
Defined Benefit Plan Disclosure [Line Items]
Target allocation of plan assets		100.00%
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Employer contributions		$ 419	$ 421
United Airlines, Inc. \| Other Postretirement Benefits \| Scenario Forecast
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 109
United Airlines, Inc. \| Pension Benefits \| Scenario Forecast
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 420

Effect of One-Percentage-Point Change in Assumed Health Care Trend Rate (Detail) - Other Postretirement Benefits $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Effect on total service and interest cost, 1% Increase	$ 11
Effect on post retirement benefit obligation, 1% Increase	170
Effect on total service and interest cost,1% Decrease	(8)
Effect on post retirement benefit obligation, 1% Decrease	$ (149)

Pension and Other Postretirement Plan Assets (Detail) - USD ($)
$ in Millions

Dec. 31, 2017

Dec. 31, 2016

Dec. 31, 2015

Pension Benefits

$ 3,932

$ 3,355

$ 2,975

Pension Benefits | Equity Securities

^[1]

2,211

1,843

1,406

1,173

Pension Benefits | Fixed-Income Securities

^[1]

1,004

832

1,470

1,298

Pension Benefits | Alternatives

^[1]

618

463

637

586

Pension Benefits | Other Investments

^[1]

498

452

419

298