Leases - Summary of the Components of Net Lease Cost (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Leases [Abstract]
Operating lease cost	$ 2,670	$ 2,720
Finance lease cost:
Amortization of right-of-use assets	56	38
Interest on lease liabilities	58	44
Total finance lease costs	114	82
Short-term lease costs	22	24
Variable lease costs	599	581
Less: sublease income	55	50
Total rental expense, net	$ 3,350	$ 3,357