Quarterly report pursuant to Section 13 or 15(d)

Stock-Based Compensation (Details 4)

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Stock-Based Compensation (Details 4) - Restricted Stock
 3 Months Ended 6 Months Ended
Jun. 30, 2015
item
$ / shares
shares
Mar. 31, 2015
$ / shares
shares
Jun. 30, 2015
item
$ / shares
shares
Stock-Based compensation      
Award vesting rights proportion (as a percent)     33.00%
Number of vesting rights anniversaries | item 3   3
Number of Shares      
Balance at the beginning of the period (in shares) 941,411 1,098,674 1,098,674
Granted (in shares)   207,531  
Vested (in shares)   (358,166)  
Forfeited (in shares) (5,869) (6,628)  
Balance at the end of the period (in shares) 935,542 941,411 935,542
Weighted-Average Grant Date Fair Value per Share      
Balance at the beginning of the period (in dollars per share) | $ / shares $ 38.61 $ 33.72 $ 33.72
Granted (in dollars per share) | $ / shares   53.89  
Vested (in dollars per share) | $ / shares   20.44  
Forfeited (in dollars per share) | $ / shares 45.98 33.53  
Balance at the end of the period (in dollars per share) | $ / shares $ 43.18 $ 38.61 $ 43.18