Leases - Assets and Liabilities (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Leases [Abstract]
Operating lease right-of-use assets	$ 2,609	$ 0
Accounts payable and other current liabilities	461
Operating lease liabilities	1,809	$ 0
Total operating lease liabilities	2,270
Property and equipment	795
Accumulated depreciation	(323)
Property and equipment, net	472
Long-term debt due within one year	48
Long-term debt	361
Total finance lease liabilities	$ 409