Leases - Supplemental Cash Flow Information (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows from financing leases	$ 3
Operating cash flows from operating leases	52
Financing cash flows from financing leases	41
Right-of-use assets obtained in exchange for lease obligations:
Operating lease liabilities	474
Finance lease liabilities	$ 83