Leases - Supplemental Cash Flow Information Related to Leases (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Cash paid for amounts included in the measurement of lease liabilities:
Operating cash flows for operating leases	$ 285	$ 279
Operating cash flows for finance leases	24	5
Financing cash flows for finance leases	$ 20	$ 18	[1]

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2016-02, Leases (Topic 842). See Note 1 to the financial statements contained in Part I, Item 1 of this report for additional information.