Leases - Lease Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease cost	$ 198	$ 207
Finance lease cost
Amortization of right-of-use assets	150	40
Interest on lease liabilities	21	8
Total lease cost	369	255
Variable lease cost included within operating lease cost	50	$ 56
Accelerated amortization expense included within finance lease expense	$ 62