SCHEDULE 13G

Amendment No. 1 
UAL Corporation 
Common Stock 
Cusip #902549807 


Cusip #902549807 
Item 1:	Reporting Person - FMR LLC
Item 4:	Delaware
Item 5:	216,266 
Item 6:	0 
Item 7:	12,092,088 
Item 8:	0 
Item 9:	12,092,088 
Item 11:	10.246% 
Item 12:	    HC


Cusip #902549807  
Item 1:	Reporting Person - Edward C. Johnson 3d 
Item 4:	United States of America
Item 5:	0 
Item 6:	0 
Item 7:	12,092,088 
Item 8:	0 
Item 9:	12,092,088 
Item 11:	10.246% 
Item 12:	IN 




	SCHEDULE 13G - TO BE INCLUDED IN 
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)

This Amendment to Schedule 13G filed by FMR LLC amends 
the statement on Schedule 13G previously filed by FMR 
Corp., the predecessor of FMR LLC.


Item 1(a).	Name of Issuer:	

		UAL Corporation 

Item 1(b).	Name of Issuer's Principal Executive Offices:

		John Lakosil - WHQLD
		77 W Wacker 
		Chicago, IL  60601
		  

Item 2(a).	Name of Person Filing: 

		FMR LLC

Item 2(b).	Address or Principal Business Office or, if None, 
Residence:

		82 Devonshire Street, Boston, 
Massachusetts  02109

Item 2(c).	Citizenship:

		Not applicable

Item 2(d).	Title of Class of Securities:

		Common Stock 

Item 2(e).	CUSIP Number:  

		902549807 

Item 3.	This statement is filed pursuant to Rule 13d-1(b) or 13d-2(b) 
and the person filing, FMR LLC, is a parent holding company 
in accordance with Section 240.13d-1(b)(ii)(G).  (Note:  See 
Item 7).

Item 4.	Ownership

	(a)	Amount Beneficially Owned:	12,092,088 

	(b)	Percent of Class:	10.246% 

	(c)	Number of shares as to which such 
person has:  

	(i)	sole power to vote or to direct 
the vote:	216,266 

	(ii)	shared power to vote or to 
direct the vote:	0 

	(iii)	sole power to dispose or to 
direct the disposition of:	12,092,088 

	(iv)	shared power to dispose or to 
direct the disposition of:	0 



Item 5.	Ownership of Five Percent or Less of a Class.

	Not applicable.

Item 6.	Ownership of More than Five Percent on Behalf of Another 
Person.

	Various persons have the right to receive or the power to 
direct the receipt of dividends from, or the proceeds from the 
sale of, the Common Stock of UAL Corporation.  No one 
person's interest in the Common Stock of UAL Corporation is 
more than five percent of the total outstanding Common 
Stock.

Item 7.	Identification and Classification of the Subsidiary Which 
Acquired the Security Being Reported on By the Parent 
Holding Company.

	See attached Exhibit A. 

Item 8.	Identification and Classification of Members of 
the Group.

	Not applicable.  See attached Exhibit A. 

Item 9.	Notice of Dissolution of Group.

	Not applicable.

Item 10.	Certification.

	By signing below I certify that, to the best of my knowledge 
and belief, the securities referred to above were acquired in the 
ordinary course of business and were not acquired for the 
purpose of and do not have the effect of changing or 
influencing the control of the issuer of such securities and 
were not acquired in connection with or as a participant in any 
transaction having such purpose or effect.

Signature

	After reasonable inquiry and to the best of my knowledge and 
belief, I certify that the information set forth in this statement 
is true, complete and correct. 

February 13, 2008 			
Date

/s/Eric D. Roiter			
Signature

Eric D. Roiter			
Duly authorized under Power of Attorney
dated December 30, 1997 by and on behalf of FMR Corp. 
(now known as FMR LLC) and its direct and indirect 
subsidiaries


	SCHEDULE 13G - TO BE INCLUDED IN 
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)



	Pursuant to the instructions in Item 7 of Schedule 13G, 
Fidelity Management & Research Company ("Fidelity"), 82 
Devonshire Street, Boston, Massachusetts 02109, a wholly-
owned subsidiary of FMR LLC and an investment adviser 
registered under Section 203 of the Investment Advisers Act 
of 1940, is the beneficial owner of 11,834,222 shares or 
10.027% of the Common Stock outstanding of UAL 
Corporation  ("the Company") as a result of acting as 
investment adviser to various investment companies registered 
under Section 8 of the Investment Company Act of 1940. The 
number of shares of Common Stock of UAL Corporation 
owned by the investment companies at December 31, 2007 
included 1,690,350 shares of Common Stock resulting from 
the assumed conversion of $58,890,000 principal amount of 
UAL CORP CONV 4.5% 6/30/21 (28.7035 shares of 
Common Stock for each $1,000 principal amount of 
debenture).

	Edward C. Johnson 3d and FMR LLC, through its 
control of Fidelity, and the funds each has sole power to 
dispose of the 11,834,222 shares owned by the Funds.

	Members of the family of Edward C. Johnson 3d, 
Chairman of FMR LLC, are the predominant owners, directly 
or through trusts, of Series B voting common shares of FMR 
LLC, representing 49% of the voting power of FMR LLC.  
The Johnson family group and all other Series B shareholders 
have entered into a shareholders' voting agreement under 
which all Series B voting common shares will be voted in 
accordance with the majority vote of Series B voting common 
shares.  Accordingly, through their ownership of voting 
common shares and the execution of the shareholders' voting 
agreement, members of the Johnson family may be deemed, 
under the Investment Company Act of 1940, to form a 
controlling group with respect to FMR LLC.

	Neither FMR LLC nor Edward C. Johnson 3d, 
Chairman of FMR LLC, has the sole power to vote or direct 
the voting of the shares owned directly by the Fidelity Funds, 
which power resides with the Funds' Boards of Trustees.  
Fidelity carries out the voting of the shares under written 
guidelines established by the Funds' Boards of Trustees.

	Pyramis Global Advisors Trust Company ("PGATC"), 
53 State Street, Boston, Massachusetts, 02109, an indirect 
wholly-owned subsidiary of FMR LLC and a bank as defined 
in Section 3(a)(6) of the Securities Exchange Act of 1934, is 
the beneficial owner of 240,566 shares or 0.204% of the 
outstanding Common Stock of the UAL Corporation as a 
result of its serving as investment manager of institutional 
accounts owning such shares. The number of shares of 
Common Stock of UAL Corporation owned by the 
institutional account(s) at December 31, 2007 included 51,666 
shares of Common Stock resulting from the assumed 
conversion of $1,800,000 principal amount of UAL CORP 
CONV 4.5% 6/30/21 (28.7035 shares of Common Stock for 
each $1,000 principal amount of debenture).

	Edward C. Johnson 3d and FMR LLC, through its 
control of Pyramis Global Advisors Trust Company, each has 
sole dispositive power over 240,566 shares and sole power to 
vote or to direct the voting of 198,266 shares of Common 
Stock owned by the institutional accounts managed by 
PGATC as reported above.

	Fidelity International Limited ("FIL"), Pembroke Hall, 
42 Crow Lane, Hamilton, Bermuda, and various foreign-based 
subsidiaries provide investment advisory and management 
services to a number of non-U.S. investment companies and 
certain institutional investors.  FIL, which is a qualified 
institution under section 240.13d-1(b)(1) pursuant to an SEC 
No-Action letter dated October 5, 2000, is the beneficial 
owner of 17,300 shares or 0.015% of the Common Stock 
outstanding of the Company. 
	Partnerships controlled predominantly by members of 
the family of Edward C. Johnson 3d, Chairman of FMR LLC 
and FIL, or trusts for their benefit, own shares of FIL voting 
stock with the right to cast approximately 47% of the total 
votes which may be cast by all holders of FIL voting stock.  
FMR LLC and FIL are separate and independent corporate 
entities, and their Boards of Directors are generally composed 
of different individuals.

	FMR LLC and FIL are of the view that they are not 
acting as a "group" for purposes of Section 13(d) under the 
Securities Exchange Act of 1934 (the "1934" Act) and that 
they are not otherwise required to attribute to each other the 
"beneficial ownership" of securities "beneficially owned" by 
the other corporation within the meaning of Rule 13d-3 
promulgated under the 1934 Act.  Therefore, they are of the 
view that the shares held by the other corporation need not be 
aggregated for purposes of Section 13(d). However, FMR 
LLC is making this filing on a voluntary basis as if all of the 
shares are beneficially owned by FMR LLC and FIL on a joint 
basis.



	SCHEDULE 13G - TO BE INCLUDED IN 
STATEMENTS
	FILED PURSUANT TO RULE 13d-1(b) or 13d-2(b)
	RULE 13d-1(f)(1)  AGREEMENT

	The undersigned persons, on February 13, 2008, agree 
and consent to the joint filing on their behalf of this Schedule 
13G in connection with their beneficial ownership of the 
Common Stock of UAL Corporation at December 31, 2007.

	FMR LLC

	By /s/ Eric D. Roiter			
	Eric D. Roiter
	Duly authorized under Power of Attorney dated 
December 30, 1997, by and on behalf of FMR Corp. (now 
known as FMR LLC) and its direct and indirect subsidiaries

	Edward C. Johnson 3d

	By /s/ Eric D. Roiter			
	Eric D. Roiter
	Duly authorized under Power of Attorney dated 
December 30, 1997, by and on behalf of Edward C. Johnson 
3d

	Fidelity Management & Research Company

	By /s/ Eric D. Roiter			
	Eric D. Roiter
	Senior V.P. and General Counsel