Schedule II - Valuation and Qualifying Accounts (Details) - UAL and United - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Allowance for doubtful accounts
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	$ 8	$ 7	$ 10
Additions Charged to Costs and Expenses	17	17	20
Deductions	16	16	23
Other	0	0	0
Balance at End of Period	9	8	7
Obsolescence allowance-spare parts
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	412	354	295
Additions Charged to Costs and Expenses	76	73	75
Deductions	63	15	17
Other	0	0	1
Balance at End of Period	425	412	354
Valuation allowance for deferred tax assets
SEC Schedule, 12-09, Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	59	63	68
Additions Charged to Costs and Expenses	0	2	11
Deductions	1	6	27
Other	0	0	11
Balance at End of Period	$ 58	$ 59	$ 63