Valuation and Qualifying Accounts (Detail) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation allowance for deferred tax assets
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		$ 68	$ 48	$ 4,751
Additions Charged to Costs and Expenses		11	47
Deductions	[1]	27	27	4,703
Other		11
Balance at End of Period		63	68	48
UAL and United | Allowance for doubtful accounts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		10	18	22
Additions Charged to Costs and Expenses		20	18	25
Deductions	[1]	23	26	29
Balance at End of Period		7	10	18
UAL and United | Obsolescence allowance-spare parts
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		295	235	169
Additions Charged to Costs and Expenses		75	61	38
Deductions	[1]	17	16
Other		1	15	28
Balance at End of Period		354	295	235
United Airlines, Inc. | Valuation allowance for deferred tax assets
Valuation and Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of Period		68	48	4,721
Additions Charged to Costs and Expenses		11	47
Deductions	[1]	27	27	4,673
Other		11
Balance at End of Period		$ 63	$ 68	$ 48

[1] Deduction from reserve for purpose for which reserve was created.