Estimated Fair Value of Notes Based on Quoted Market Prices (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
5.75 %Senior Notes Due 2022 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of debt	$ 789	$ 668
5.625% Senior Notes Due 2024 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of debt	1,044	831
Senior Notes Due 2026 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of debt	$ 714	$ 542