Schedule II - Valuation and Qualifying Accounts (Details) - UAL and United - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Allowance for doubtful accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	$ 7	$ 10	$ 18
Additions Charged to Costs and Expenses	17	20	18
Deductions	16	23	26
Other	0	0	0
Balance at End of Period	8	7	10
Obsolescence allowance-spare parts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	354	295	235
Additions Charged to Costs and Expenses	73	75	61
Deductions	15	17	16
Other	0	1	15
Balance at End of Period	412	354	295
Valuation allowance for deferred tax assets
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period	63	68	48
Additions Charged to Costs and Expenses	2	11	47
Deductions	6	27	27
Other	0	11	0
Balance at End of Period	$ 59	$ 63	$ 68