Leases and Capacity Purchase Agreements - Components of Lease Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease cost	$ 958	$ 933	$ 1,038
Variable and short-term lease cost	2,291	1,968	2,548
Amortization of finance lease assets	89	88	68
Interest on finance lease liabilities	16	16	85
Sublease income	(26)	(23)	(32)
Total lease cost	$ 3,328	$ 2,982	$ 3,707