Leases - Lease Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Lessee, Lease, Description [Line Items]
Finance lease cost, amortization of leased assets	$ 131	$ 110	$ 100
Finance lease cost, interest of lease liabilities	32	29	22
Operating lease cost	1,019	1,013	994
Short-term lease cost	264	500	458
Variable lease cost	1,406	1,456	1,427
Total lease cost	2,852	3,108	3,001
Regional carrier
Lessee, Lease, Description [Line Items]
Operating lease cost	187	174	150
Short-term lease cost			18
Variable lease cost	$ 50	$ 64	$ 48