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Debt and Interest Costs - Debt refinancing (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Debt Refinancing and Other Costs: (Abstract)      
Refinance of bridge and revolving credit facilities   $ (30) $ (16)
Economic hedge - interest rate risk   (20) (16)
Individually insignificant items $ (3) (3)  
Debt refinancing and other costs $ (3) (109) (117)
Senior Notes 3.5 Percent And 4.375 Percent      
Debt Refinancing and Other Costs: (Abstract)      
Interest expense on 3.5% and 4.375% senior notes   (49) (5)
Senior Notes 3.5 Percent [Member]      
Debt Refinancing and Other Costs: (Abstract)      
Interest rate (as a percent) 3.50%    
Senior Notes 4.375 Percent [Member]      
Debt Refinancing and Other Costs: (Abstract)      
Interest rate (as a percent) 4.375%    
Senior Notes 5.75 Percent Due May 2021 And 6.75 Percent Due September 2020      
Debt Refinancing and Other Costs: (Abstract)      
Redemption of 6.75% and 5.75% senior notes, due September 2020 and May 2021, respectively, and refinance of senior credit facilities     (57)
5.75% Senior Notes, due May 2021      
Debt Refinancing and Other Costs: (Abstract)      
Interest rate (as a percent) 5.75%    
6.75% Senior Notes, due September 2020      
Debt Refinancing and Other Costs: (Abstract)      
Interest rate (as a percent) 6.75%    
Bridge Facility      
Debt Refinancing and Other Costs: (Abstract)      
Amortization of unsecured, committed bridge facility financing fees   $ (7) $ (23)