Leases - Supplemental Cash Flow (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from operating leases	$ 572	$ 388
Operating cash flows from finance leases	7	5
Financing cash flows from finance leases	209	267
Lease liabilities arising from obtaining right-of-use assets:
Operating leases	689	2,203
Finance leases	$ 33	$ 75