Leases - Lease Expense (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating and short-term lease cost	$ 729	$ 677
Finance lease cost:
Amortization of right-of-use assets	36	44
Interest on lease liability	12	12
Total finance lease cost	48	56
Total lease cost	$ 777	$ 733