Leases - Supplemental Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating cash flows for operating leases	$ 566	$ 665
Operating cash flows for finance leases	14	14
Financing cash flows for finance leases	40	32
Operating lease right-of-use assets obtained in exchange for new operating lease liabilities	375	358
Right-of-use assets obtained in exchange for new finance lease liabilities	$ 124	$ 14