Schedule II - Valuation and Qualifying Accounts (Details) - UAL and United - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Allowance for doubtful accounts
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	$ 78	$ 9	$ 8
Additions Charged to Costs and Expenses	3	70	17
Deductions	53	16	16
Other	0	15	0
Balance at End of Period	28	78	9
Obsolescence allowance-spare parts
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	478	425	412
Additions Charged to Costs and Expenses	79	88	76
Deductions	11	35	63
Other	0	0	0
Balance at End of Period	546	478	425
Allowance for credit losses - notes receivable
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	522	0
Additions Charged to Costs and Expenses	1	518
Deductions	0	0
Other	99	4
Balance at End of Period	622	522	0
Valuation allowance for deferred tax assets
SEC Schedule, 12-09, Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Beginning of Period	247	58	59
Additions Charged to Costs and Expenses	(38)	197	0
Deductions	0	8	1
Other	1	0	0
Balance at End of Period	$ 210	$ 247	$ 58