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	(3,331)						(3,331)		(3,330)				(3,330)
Other comprehensive income (loss)	(541)							(541)	(541)							(541)
Dividend to UAL									(342)			(12)	(330)
Stock settled share-based compensation	42				42				42			42
Issuance of common stock (in shares)				43,700,000
Issuance of common stock	1,135				1,135				1,135								1,135
Warrants issued	57				57
Repurchases of common stock (in shares)				(4,400,000)
Repurchases of common stock	(342)					(342)
Stock issued for share-based awards, net of shares withheld for tax (in shares)				500,000
Stock issued for share-based awards, net of shares withheld for tax	(17)				(56)	42	(3)
Other									46								46
Balance (in shares) at Jun. 30, 2020				291,000,000.0
Balance at Jun. 30, 2020	8,517			$ 3	7,307	(3,899)	6,365	(1,259)	8,485			30	8,676			(1,259)	1,038
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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(1,627)						(1,627)		(1,626)				(1,626)
Other comprehensive income (loss)	(488)							(488)	(488)							(488)
Stock settled share-based compensation	20				20				20			20
Issuance of common stock (in shares)				43,700,000
Issuance of common stock	1,135				1,135				1,135								1,135
Warrants issued	57				57
Stock issued for share-based awards, net of shares withheld for tax	2				(1)	2	1
Other									64								64
Balance (in shares) at Jun. 30, 2020				291,000,000.0
Balance at Jun. 30, 2020	$ 8,517			$ 3	7,307	(3,899)	6,365	(1,259)	$ 8,485			30	8,676			(1,259)	1,038
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
Balance (in shares) at Mar. 31, 2021				323,600,000
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
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	(1,791)						(1,791)		(1,791)				(1,791)
Other comprehensive income (loss)	25							25	25							25
Stock settled share-based compensation	100				100				100			100
Issuance of common stock (in shares)				11,000,000.0
Issuance of common stock	532				532				532								532
Warrants issued	99				99
Stock issued for share-based awards, net of shares withheld for tax (in shares)				800,000
Stock issued for share-based awards, net of shares withheld for tax	$ (21)				(55)	65	(31)
Other									$ 78								78
Balance (in shares) at Jun. 30, 2021	323,610,765			323,600,000					1,000
Balance at Jun. 30, 2021	$ 4,904			$ 4	9,042	(3,832)	804	(1,114)	$ 4,872			185	3,148			(1,114)	2,653
Balance (in shares) at Mar. 31, 2021				323,600,000
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(434)						(434)		(434)				(434)
Other comprehensive income (loss)	12							12	12							12
Stock settled share-based compensation	68				68				68			68
Warrants issued	52				52
Stock issued for share-based awards, net of shares withheld for tax	$ 0				(1)	2	(1)
Other									$ 51								51
Balance (in shares) at Jun. 30, 2021	323,610,765			323,600,000					1,000
Balance at Jun. 30, 2021	$ 4,904			$ 4	$ 9,042	$ (3,832)	$ 804	$ (1,114)	$ 4,872			$ 185	$ 3,148			$ (1,114)	$ 2,653

[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.