Leases - Supplemental Cash Flow Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows paid for operating leases	$ 2,724	$ 2,701
Operating cash flows paid for interest portion of finance leases	58	44
Financing cash flows paid for principal portion of finance leases	34	26
Right-of-use assets obtained in exchange for lease obligations:
Operating leases	1,679	1,824
Finance leases	$ 313	$ 283