Leases - Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$ 155	$ 139
Operating cash flows from finance leases	21	8
Financing cash flows from finance leases	187	37
Right-of-use assets obtained in exchange for lease liabilities:
Operating leases	46	441
Finance leases	$ 399	$ 288