Leases and Capacity Purchase Agreements - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) Aircraft aircraft	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Lessee, Lease, Description [Line Items]
Additional leases that have not yet commenced | $		$ 500
United Airlines, Inc. | Bombardier CRJ-550
Lessee, Lease, Description [Line Items]
Number of aircraft operated under CPA		54
Term of CPA		10 years
United Airlines, Inc. | Embraer E175 LL
Lessee, Lease, Description [Line Items]
Number of aircraft operated under CPA		20
United Airlines, Inc. | Embraer E175
Lessee, Lease, Description [Line Items]
Number of aircraft operated under CPA		42
Term of CPA		5 years
United Airlines, Inc. | Cash Collateralized Letters of Credit
Lessee, Lease, Description [Line Items]
Number of regional aircraft | Aircraft		581
Mainline Aircraft
Lessee, Lease, Description [Line Items]
Number of aircraft subject to operating leases		114
Number of aircraft subject to finance leases		28
Regional Aircraft
Lessee, Lease, Description [Line Items]
Number of aircraft subject to operating leases		325
Number of aircraft subject to finance leases		17
Regional Aircraft | United Airlines, Inc. | Capacity Purchase Agreements
Lessee, Lease, Description [Line Items]
Expenses | $		$ 1,000	$ 979	$ 907
Accounts payable | $		$ 69	$ 53
Minimum | Mainline Aircraft
Lessee, Lease, Description [Line Items]
Initial term of operating leases		1 month
Minimum | Non-Aircraft
Lessee, Lease, Description [Line Items]
Initial term of operating leases		1 month
Maximum
Lessee, Lease, Description [Line Items]
Lease terms of additional leases that have not yet commenced (up to)		13 years
Maximum | Mainline Aircraft
Lessee, Lease, Description [Line Items]
Initial term of operating leases		10 years
Maximum | Non-Aircraft
Lessee, Lease, Description [Line Items]
Initial term of operating leases		34 years
Forecast | Capacity Purchase Agreements
Lessee, Lease, Description [Line Items]
Scheduled block hours increase (decrease) percentage	10.00%
Change in cash obligation | $	$ 202