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	Retained Earnings Adoption of New Accounting Standard	[1]	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	$ (17)		$ (718)	$ 11,492	$ (17)		$ 0	$ 12,353	$ (17)		$ (718)	$ (143)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(5,172)						(5,172)				(5,171)				(5,171)
Other comprehensive income (loss)	(291)									(291)	(291)							(291)
Dividend to UAL											(342)			(12)	(330)
Stock-settled share-based compensation	80				80						80			80
Issuance of common stock (in shares)				43,700,000
Issuance of common stock	1,135				1,135						1,135								1,135
Warrants issued	97				97
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	(18)				(58)	43	(3)
Other											86								86
Balance (in shares) at Sep. 30, 2020				291,000,000.0
Balance at Sep. 30, 2020	7,003			$ 3	7,383	(3,898)	4,524			(1,009)	6,972			68	6,835			(1,009)	1,078
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	(1,841)						(1,841)				(1,841)				(1,841)
Other comprehensive income (loss)	250									250	250							250
Stock-settled share-based compensation	38				38						38			38
Warrants issued	40				40
Stock issued for share-based awards, net of shares withheld for tax	(1)				(2)	1	0
Other											40								40
Balance (in shares) at Sep. 30, 2020				291,000,000.0
Balance at Sep. 30, 2020	$ 7,003			$ 3	7,383	(3,898)	4,524			(1,009)	$ 6,972			68	6,835			(1,009)	1,078
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 income (loss)	(1,318)						(1,318)				(1,317)				(1,317)
Other comprehensive income (loss)	15									15	15							15
Stock-settled share-based compensation	169				169						169			169
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)				1,000,000.0
Stock issued for share-based awards, net of shares withheld for tax	$ (26)				(72)	83	(37)
Other											$ 72								72
Balance (in shares) at Sep. 30, 2021	323,806,610			323,800,000							1,000
Balance at Sep. 30, 2021	$ 5,431			$ 4	9,094	(3,814)	1,271			(1,124)	$ 5,399			254	3,622			(1,124)	2,647
Balance (in shares) at Jun. 30, 2021				323,600,000
Balance at Jun. 30, 2021	4,904			$ 4	9,042	(3,832)	804			(1,114)	4,872			185	3,148			(1,114)	2,653
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)	473						473				474				474
Other comprehensive income (loss)	(10)									(10)	(10)							(10)
Stock-settled share-based compensation	69				69						69			69
Stock issued for share-based awards, net of shares withheld for tax	$ (5)				(17)	18	(6)
Other											$ (6)								(6)
Balance (in shares) at Sep. 30, 2021	323,806,610			323,800,000							1,000
Balance at Sep. 30, 2021	$ 5,431			$ 4	$ 9,094	$ (3,814)	$ 1,271			$ (1,124)	$ 5,399			$ 254	$ 3,622			$ (1,124)	$ 2,647

[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.