Leases - Supplemental Cash Flow (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$ 276	$ 188
Operating cash flows from finance leases	3	2
Financing cash flows from finance leases	100	125
Lease liabilities arising from obtaining right-of-use assets:
Operating leases	304	1,383
Finance leases	$ 25	$ 35