Leases and Capacity Purchase Agreements - Supplemental Cash Flow Information Related to Leases (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows for operating leases	$ 788	$ 902	$ 1,078
Operating cash flows for finance leases	20	70	53
Financing cash flows for finance leases	$ 66	$ 151	$ 79