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Stock-Based Compensation (PBRSU) (Details) - PBRSU [Member] - $ / shares
3 Months Ended
Mar. 31, 2017
Mar. 31, 2016
Dec. 31, 2016
Number of Shares      
Number of units unvested (in units) 692,000   738,000
Granted (in units) 288,331 174,213  
Vested (in units) 227,000    
Forfeited (in units) (107,000)    
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) 36.86    
Unvested units outstanding, Ending Balance (usd per unit) $ 36.19