Leases - Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating cash flows for operating leases	$ 350	$ 387
Operating cash flows for finance leases	13	11
Financing cash flows for finance leases	18	5
Operating lease right-of-use assets obtained in exchange for new operating lease liabilities	151	206
Right-of-use assets obtained in exchange for new finance lease liabilities	$ 100	$ 12