Income Taxes - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Taxes [Line Items]
Valuation allowance	$ 197	$ (4)	$ (3)
Unrecognized tax benefits	57	53	39
Unrecognized tax benefits that would impact effective tax rate if recognized	57
Capital loss carryforwards, valuation allowance	185
United Airlines, Inc.
Income Taxes [Line Items]
Valuation allowance	197	$ (4)	$ (3)
Federal
Income Taxes [Line Items]
Tax credits	42
Federal | United Airlines, Inc.
Income Taxes [Line Items]
NOL carryforwards	11,000
Tax effected amount of NOLs	2,300
NOLs without an expiration date	$ 9,400
Federal | Minimum
Income Taxes [Line Items]
Tax credit expiration period	1 year
Federal | Maximum
Income Taxes [Line Items]
Tax credit expiration period	18 years
Federal | 2026 | United Airlines, Inc.
Income Taxes [Line Items]
NOL carryforwards	$ 100
Federal | 2028 | United Airlines, Inc.
Income Taxes [Line Items]
NOL carryforwards	500
Federal | 2029 | United Airlines, Inc.
Income Taxes [Line Items]
NOL carryforwards	400
Federal | 2032 | United Airlines, Inc.
Income Taxes [Line Items]
NOL carryforwards	200
Federal | 2033 | United Airlines, Inc.
Income Taxes [Line Items]
NOL carryforwards	400
State
Income Taxes [Line Items]
Tax credits	33
NOL and tax credit carryforwards, valuation allowance	62
State | United Airlines, Inc.
Income Taxes [Line Items]
NOL carryforwards	3,700
Tax effected amount of NOLs	$ 200
State | Minimum
Income Taxes [Line Items]
Tax credit expiration period	1 year
State | Minimum | United Airlines, Inc.
Income Taxes [Line Items]
NOL expiration period	5 years
State | Maximum
Income Taxes [Line Items]
Tax credit expiration period	11 years
State | Maximum | United Airlines, Inc.
Income Taxes [Line Items]
NOL expiration period	20 years