Schedule II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Allowance for credit losses - receivables:
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	$ 11	$ 28	$ 78
Additions Charged to Costs and Expenses	27	22	3
Deductions	23	39	53
Other	3	0	0
Balance at End of Period	18	11	28
Allowance for credit losses - receivables: | United Airlines, Inc.
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	11	28	78
Additions Charged to Costs and Expenses	27	22	3
Deductions	23	39	53
Other	3	0	0
Balance at End of Period	18	11	28
Obsolescence allowance—spare parts:
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	610	546	478
Additions Charged to Costs and Expenses	102	73	79
Deductions	23	9	11
Other	0	0	0
Balance at End of Period	689	610	546
Obsolescence allowance—spare parts: | United Airlines, Inc.
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	610	546	478
Additions Charged to Costs and Expenses	102	73	79
Deductions	23	9	11
Other	0	0	0
Balance at End of Period	689	610	546
Allowance for credit losses - investments in affiliates and other:
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	21	622	522
Additions Charged to Costs and Expenses	20	20	1
Deductions	0	539	0
Other	(3)	(82)	99
Balance at End of Period	38	21	622
Allowance for credit losses - investments in affiliates and other: | United Airlines, Inc.
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	21	622	522
Additions Charged to Costs and Expenses	20	20	1
Deductions	0	539	0
Other	(3)	(82)	99
Balance at End of Period	38	21	622
Valuation allowance for deferred tax assets:
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	199	210	247
Additions Charged to Costs and Expenses	(21)	(10)	(38)
Deductions	0	0	0
Other	1	(1)	1
Balance at End of Period	179	199	210
Valuation allowance for deferred tax assets: | United Airlines, Inc.
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	199	210	247
Additions Charged to Costs and Expenses	(21)	(10)	(38)
Deductions	0	0	0
Other	1	(1)	1
Balance at End of Period	$ 179	$ 199	$ 210