Leases (Balance Sheet Amounts) (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Operating
Operating lease assets	$ 922	$ 0
Accrued expenses and other current liabilities, operating leases	146
Noncurrent operating lease liabilities	858	0
Total lease liabilities	1,004
Finance
Property and equipment, net	34
Accrued expenses and other current liabilities, finance leases	12
Long-term debt	26
Total lease liabilities	$ 38	$ 43