Income Taxes - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Income Taxes [Line Items]
Capital loss carryforwards, valuation allowance	$ 150
Unrecognized tax benefits	66	$ 58	$ 55
Federal
Income Taxes [Line Items]
Tax credits	50
Federal | United Airlines, Inc.
Income Taxes [Line Items]
NOL carryforwards	12,000
Tax effected amount of NOLs	2,500
NOLs without an expiration date	$ 11,600
Federal | Minimum
Income Taxes [Line Items]
Tax credit expiration period	1 year
Federal | Maximum
Income Taxes [Line Items]
Tax credit expiration period	20 years
Federal | 2029 | United Airlines, Inc.
Income Taxes [Line Items]
NOL carryforwards	$ 200
Federal | 2033 | United Airlines, Inc.
Income Taxes [Line Items]
NOL carryforwards	200
State
Income Taxes [Line Items]
Tax credits	56
NOL and tax credit carryforwards, valuation allowance	29
State | United Airlines, Inc.
Income Taxes [Line Items]
NOL carryforwards	3,400
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	1 year
State | Maximum
Income Taxes [Line Items]
Tax credit expiration period	15 years
State | Maximum | United Airlines, Inc.
Income Taxes [Line Items]
NOL expiration period	20 years