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Income Tax Provision Differed from Amounts Computed at Statutory Federal Income Tax Rate and Significant Components (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Taxes [Line Items]      
Income tax provision at statutory rate $ 1,050 $ 1,337 $ 1,477
State income taxes, net of federal income tax 29 38 38
Foreign tax rate differential (43)    
Foreign income taxes 3 3 4
Nondeductible employee meals 17 16 15
Impact of Tax Act (192)    
Income tax adjustment from AOCI   180  
State rate change 12 (12)  
Valuation allowance (16) 20 (4,662)
Other, net 8 (26) 7
Income tax expense (benefit) 868 1,556 (3,121)
Current (77) (92) 56
Deferred 945 1,648 (3,177)
Income tax expense (benefit) 868 1,556 (3,121)
United Airlines, Inc.      
Income Taxes [Line Items]      
Income tax provision at statutory rate 1,051 1,338 1,477
State income taxes, net of federal income tax 29 38 38
Foreign tax rate differential (43)    
Foreign income taxes 3 3 4
Nondeductible employee meals 17 16 15
Impact of Tax Act (209)    
Income tax adjustment from AOCI   180  
State rate change 12 (12)  
Valuation allowance (16) 20 (4,621)
Other, net 8 (25) 7
Income tax expense (benefit) 852 1,558 (3,080)
Current (77) (92) 56
Deferred 929 1,650 (3,136)
Income tax expense (benefit) $ 852 $ 1,558 $ (3,080)