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Employee Benefits and Share-Based Compensation - Performance Based Restricted Stock Unit Activity (Details) - 2009 Plan - Performance Shares - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Number of Shares    
Outstanding at beginning of period (in shares) 225  
Granted (in shares) 110  
Vested (in shares) (32)  
Canceled (in shares) (1)  
Outstanding and unvested at end of period (in shares) 302  
Weighted-Average Grant Date Fair Value Per Share    
Outstanding and unvested (in dollars per share) $ 31.18  
Granted (in dollars per share) 38.03 $ 32.37
Vested (in dollars per share) 32.37  
Canceled (in dollars per share) 43.20  
Outstanding and unvested (in dollars per share) $ 33.51  
Unrecognized compensation cost $ 5.4  
Weighted average period of compensation cost not yet recognized (in years) 1 year 6 months