Income Taxes - Income Tax Provision (Benefit) Differed from Amounts Computed at the Statutory Federal Income Tax Rate and Significant Components (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016		Sep. 30, 2015
Income Taxes [Line Items]
Income tax provision at statutory rate	$ 558	$ 1,064		$ 1,320
State income taxes, net of federal income tax benefit	29	30		38
Foreign tax rate differential	(84)	(43)		0
Global intangible low-taxed income	4	0		0
Foreign income taxes	2	3		3
Nondeductible employee meals	12	17		16
Impact of Tax Act	(5)	(179)		0
Income tax adjustment from AOCI	0	0		180
State rate change	3	12		(12)
Valuation allowance	(3)	(16)		20
Other, net	13	8		(26)
Income tax expense (benefit)	529	896	[1]	1,539	[1]
Current	14	(77)		(92)
Deferred	515	973	[2]	1,631	[2]
Valuation allowance recorded	59	63
AOCI
Income Taxes [Line Items]
Income tax adjustment from AOCI				180
Valuation allowance recorded						$ 465
AOCI | Pension and Other Postretirement Liabilities
Income Taxes [Line Items]
Valuation allowance recorded						285
AOCI | Designated as Hedging Instrument | Fuel
Income Taxes [Line Items]
Valuation allowance recorded						$ 180
United Airlines, Inc.
Income Taxes [Line Items]
Income tax provision at statutory rate	559	1,065		1,321
State income taxes, net of federal income tax benefit	29	30		38
Foreign tax rate differential	(84)	(43)		0
Global intangible low-taxed income	4	0		0
Foreign income taxes	2	3		3
Nondeductible employee meals	12	17		16
Impact of Tax Act	(5)	(196)		0
Income tax adjustment from AOCI	0	0		180
State rate change	3	12		(12)
Valuation allowance	(3)	(16)		20
Other, net	12	7		(25)
Income tax expense (benefit)	529	879	[3]	1,541	[3]
Current	14	(77)		(92)
Deferred	515	956	[4]	$ 1,633	[4]
Valuation allowance recorded	$ 59	$ 63

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606) and Accounting Standards Update No. 2017-07, Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. See Note 1 to the financial statements contained in Part II, Item 8 of this report for additional information.

[2] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements contained in Part II, Item 8 of this report for additional information.

[3] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606) and Accounting Standards Update No. 2017-07, Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost. See Note 1 to the financial statements contained in Part II, Item 8 of this report for additional information.

[4] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements contained in Part II, Item 8 of this report for additional information.