Leases - Supplemental Cash Flow Information (Details) $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($)
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows paid for operating leases	$ 2,023
Operating cash flows paid for interest portion of finance leases	32
Financing cash flows paid for principal portion of finance leases	19
Right-of-use assets obtained in exchange for lease obligations:
Operating leases	1,203
Finance leases	$ 82