Debt - Long-tem Debt Issued (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Amount net of discount and fees	$ 5,195	$ 5,199
Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Amount net of discount and fees	5,195	5,199
$600 million, 3.05% Notes due 2027 | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 600	$ 600
Long-term debt, stated interest rate	3.05%	3.05%
Amount net of discount and fees	$ 594	$ 594
$1,000 million, 3.875% Notes due 2047 | Uncollateralized Debt [Member]
Debt Instrument [Line Items]
Long-term debt, face value	$ 1,000	$ 1,000
Long-term debt, stated interest rate	3.875%	3.875%
Amount net of discount and fees	$ 988	$ 988