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Stock-Based Compensation (PBRSU) (Details) - PBRSU [Member] - $ / shares
6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Dec. 31, 2015
Number of Shares      
Number of units unvested (in units) 758,000   659,000
Granted (in units) 278,799 222,788  
Vested (in units) 88,000    
Forfeited (in units) (91,000)    
Weighted Average Grant Date Fair Value      
Unvested units outstanding, Beginning Balance (usd per unit) $ 29.15    
Granted (usd per unit) 35.30    
Vested (usd per unit) 26.04    
Forfeited (usd per unit) 19.50    
Unvested units outstanding, Ending Balance (usd per unit) $ 34.10