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Subsequent Events (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Feb. 23, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Feb. 28, 2018
Jan. 16, 2018
Subsequent event            
Dividend declared (in dollars per share)   $ 0.24 $ 0.24 $ 0.24    
6.15% senior notes due February 2018            
Subsequent event            
Interest rate (as a percent)   6.15%        
Subsequent Event            
Subsequent event            
Dividend declared (in dollars per share) $ 0.06          
Subsequent Event | 5.75% senior unsecured notes due February 1, 2025            
Subsequent event            
Aggregate principal amount           $ 800
Interest rate (as a percent)           5.75%
Subsequent Event | 6.15% senior notes due February 2018            
Subsequent event            
Interest rate (as a percent)         6.15%