Debt Key Components of Long-term Debt (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Fair Value Inputs, Discount Rate	1.00%
Long-term debt	$ 5,475	$ 5,469
Long-term Debt
Debt Instrument [Line Items]
Debt Instrument, Unamortized Discount	$ 4
Debt Instrument, Interest Rate During Period	2.55%
Long-term Notes $2.5B
Debt Instrument [Line Items]
Fair Value Inputs, Discount Rate	99.943%
Long-term debt	$ 2,499	2,499
Long-term Notes $2.0B
Debt Instrument [Line Items]
Fair Value Inputs, Discount Rate	99.76%
Long-term debt	$ 1,996	1,996
Long-term Notes $1.0B
Debt Instrument [Line Items]
Fair Value Inputs, Discount Rate	99.925%
Long-term debt	$ 1,000	1,000
Long-term Notes $5.5B
Debt Instrument [Line Items]
Long-term debt	5,495	5,495
Debt Issuance Cost	$ (20)	$ (26)