Statements of Consolidated Stockholder's Equity - USD ($) $ in Millions		Total		United Airlines, Inc.		Adoption of New Accounting Standard		Adoption of New Accounting Standard United Airlines, Inc.	[2]	Common Stock	Additional Capital Invested	Additional Capital Invested United Airlines, Inc.	Treasury Stock	Retained Earnings	Retained Earnings United Airlines, Inc.	Retained Earnings Adoption of New Accounting Standard		Retained Earnings Adoption of New Accounting Standard United Airlines, Inc.	[2]	Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Income (Loss) United Airlines, Inc.	Accumulated Other Comprehensive Income (Loss) Adoption of New Accounting Standard	(Receivable from) Payable to Related Parties, Net United Airlines, Inc.
Balance (in shares) at Dec. 31, 2017										287,000,000.0
Balance at Dec. 31, 2017		$ 8,788		$ 8,751		$ 0				$ 3	$ 6,098	$ 1,787	$ (769)	$ 4,603	$ 8,201	$ (6)				$ (1,147)	$ (1,147)	$ 6	$ (90)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)		2,122		2,123										2,122	2,123
Other comprehensive loss		338		338																338	338
Dividend to UAL				(1,249)								(1,249)
Stock-settled share-based compensation		60		60							60	60
Repurchases of common stock (in shares)										(17,500,000)
Repurchases of common stock		(1,250)											(1,250)
Net treasury stock issued for share-based awards (in shares)										400,000
Net treasury stock issued for share-based awards		(16)									(38)		26	(4)
Other (b)				(19)											(5)						6		(20)
Balance (in shares) at Dec. 31, 2018										269,900,000
Balance at Dec. 31, 2018		10,042		10,004						$ 3	6,120	598	(1,993)	6,715	10,319					(803)	(803)		(110)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income (loss)		3,009		3,011										3,009	3,011
Other comprehensive loss		85		85																85	85
Dividend to UAL				(1,641)								(664)			(977)
Stock-settled share-based compensation		66		66							66	66
Repurchases of common stock (in shares)										(19,200,000)
Repurchases of common stock		(1,641)											(1,641)
Net treasury stock issued for share-based awards (in shares)										500,000
Net treasury stock issued for share-based awards		$ (30)									(57)		35	(8)
Other (b)				$ (33)											0						0		(33)
Balance (in shares) at Dec. 31, 2019		251,216,381		1,000						251,200,000
Balance at Dec. 31, 2019		$ 11,531		$ 11,492		$ (17)	[1]	$ (17)		$ 3	6,129	0	(3,599)	9,716	12,353	$ (17)	[1]	$ (17)		(718)	(718)		(143)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]		us-gaap:AccountingStandardsUpdate201613Member	[1]	us-gaap:AccountingStandardsUpdate201613Member	[2]
Net income (loss)		$ (7,069)		$ (7,067)										(7,069)	(7,067)
Other comprehensive loss		(421)		(421)																(421)	(421)
Dividend to UAL				(342)								(12)			(330)
Sale of common stock (in shares)										64,600,000
Sale of common stock		2,103								$ 1	2,102
Stock-settled share-based compensation		$ 97		97							97	97
Repurchases of common stock (in shares)		(4,000,000)								(4,400,000)
Repurchases of common stock		$ (342)											(342)
Net treasury stock issued for share-based awards (in shares)										400,000
Net treasury stock issued for share-based awards		(19)									(59)		44	(4)
Warrants issued		$ 97									97
Other (b)	[3]			$ 2,186																			2,186
Balance (in shares) at Dec. 31, 2020		311,845,232		1,000						311,800,000
Balance at Dec. 31, 2020		$ 5,960		$ 5,928						$ 4	$ 8,366	$ 85	$ (3,897)	$ 2,626	$ 4,939					$ (1,139)	$ (1,139)		$ 2,043
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Accounting Standards Update [Extensible List]		us-gaap:AccountingStandardsUpdate201613Member

[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.

[3] Primarily relates to equity issuances of UAL common stock.