Debt (Estimated Fair Value of Significant Financial Instruments) (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term debt	$ 265	$ 12
Long-term debt	4,748	4,402
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Short-term debt	265	12
Long-term debt	$ 4,931	$ 4,513