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Stock-Based Compensation (PBRSU) (Details) - PBRSU [Member] - $ / shares
9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Number of Shares      
Number of units unvested (in units) 640,000   738,000
Granted (in units) 288,331 285,475  
Vested (in units) (227,000)    
Forfeited (in units) (159,000)    
Weighted Average Grant Date Fair Value      
Unvested units outstanding, Beginning Balance (usd per unit) $ 34.13    
Granted (usd per unit) 39.79    
Vested (usd per unit) 31.90    
Forfeited (usd per unit) 36.48    
Unvested units outstanding, Ending Balance (usd per unit) $ 36.91