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Stock-Based Compensation (PBRSU) (Details) - PBRSU [Member] - $ / shares
6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Dec. 31, 2016
Number of Shares      
Number of units unvested (in units) 678,000   738,000
Granted (in units) 288,331 278,799  
Vested (in units) (227,000)    
Forfeited (in units) (121,000)    
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) 36.72    
Unvested units outstanding, Ending Balance (usd per unit) $ 36.62