Leases - Cash Paid for Leases (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating cash outflows—finance leases	$ 10	$ 6
Operating cash outflows—operating leases	521	553
Financing cash outflows—finance leases	$ 17	$ 21