Leases - Supplemental Cash Flow Information (Details) $ in Millions	6 Months Ended
	Jun. 30, 2019 USD ($)
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows paid for operating leases	$ 1,348
Operating cash flows paid for interest portion of finance leases	21
Financing cash flows paid for principal portion of finance leases	13
Right-of-use assets obtained in exchange for lease obligations:
Operating leases	868
Finance leases	$ 43