Pension and Other Postretirement Plans - Allocation of Plan Assets (Details) - United Airlines, Inc.	12 Months Ended
	Dec. 31, 2018
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return	9.50%
Equity securities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Percent of total	30.00%
Equity securities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Percent of total	45.00%
Fixed-income securities
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return	5.80%
Fixed-income securities | Minimum
Defined Benefit Plan Disclosure [Line Items]
Percent of total	30.00%
Fixed-income securities | Maximum
Defined Benefit Plan Disclosure [Line Items]
Percent of total	40.00%
Alternatives
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return	7.30%
Alternatives | Minimum
Defined Benefit Plan Disclosure [Line Items]
Percent of total	10.00%
Alternatives | Maximum
Defined Benefit Plan Disclosure [Line Items]
Percent of total	25.00%
Other
Defined Benefit Plan Disclosure [Line Items]
Expected long-term rate of return	7.80%
Other | Minimum
Defined Benefit Plan Disclosure [Line Items]
Percent of total	0.00%
Other | Maximum
Defined Benefit Plan Disclosure [Line Items]
Percent of total	10.00%