Statements of Consolidated Stockholders' Equity (Unaudited) - USD ($) $ in Millions	Total	Adoption of New Accounting Standard	[1]	Common Stock	Additional Capital Invested	Treasury Stock	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	United Airlines, Inc.	United Airlines, Inc. Adoption of New Accounting Standard	[2]	United Airlines, Inc. Additional Capital Invested	United Airlines, Inc. Retained Earnings	United Airlines, Inc. Retained Earnings Adoption of New Accounting Standard	[2]	United Airlines, Inc. Accumulated Other Comprehensive Income (Loss)	United Airlines, Inc. (Receivable from) Payable to Related Parties, Net
Balance (in shares) at Dec. 31, 2019				251,200,000
Balance at Dec. 31, 2019	$ 11,531	$ (17)		$ 3	$ 6,129	$ (3,599)	$ 9,716	$ (718)	$ 11,492	$ (17)		$ 0	$ 12,353	$ (17)		$ (718)	$ (143)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(1,704)						(1,704)		(1,704)				(1,704)
Other comprehensive income (loss)	(53)							(53)	(53)							(53)
Dividend to UAL									(342)			(12)	(330)
Stock settled share-based compensation	22				22				22			22
Repurchases of common stock (in shares)				(4,400,000)
Repurchases of common stock	(342)					(342)
Net treasury stock issued for share-based awards (in shares)				500,000
Net treasury stock issued for share-based awards	(19)				(55)	40	(4)
Other									(18)								(18)
Balance (in shares) at Mar. 31, 2020				247,300,000
Balance at Mar. 31, 2020	$ 9,418			$ 3	6,096	(3,901)	7,991	(771)	$ 9,380			10	10,302			(771)	(161)
Balance (in shares) at Dec. 31, 2020	311,845,232			311,800,000					1,000
Balance at Dec. 31, 2020	$ 5,960			$ 4	8,366	(3,897)	2,626	(1,139)	$ 5,928			85	4,939			(1,139)	2,043
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(1,357)						(1,357)		(1,357)				(1,357)
Other comprehensive income (loss)	13							13	13							13
Stock settled share-based compensation	32				32				32			32
Issuance of common stock (in shares)				11,000,000.0
Issuance of common stock	532				532	0			532								532
Warrants issued	47				47
Net treasury stock issued for share-based awards (in shares)				800,000
Net treasury stock issued for share-based awards	$ (21)				(54)	63	(30)
Other									$ 27								27
Balance (in shares) at Mar. 31, 2021	323,577,438			323,600,000					1,000
Balance at Mar. 31, 2021	$ 5,206			$ 4	$ 8,923	$ (3,834)	$ 1,239	$ (1,126)	$ 5,175			$ 117	$ 3,582			$ (1,126)	$ 2,602

[1] Transition adjustment due to the adoption of Accounting Standards Update No. 2016-13, Financial Instruments—Credit Losses.

[2] Transition adjustment due to the adoption of Accounting Standards Update No. 2016-13, Financial Instruments—Credit Losses.