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Employee Benefits and Share-Based Compensation - Restricted Stock Units and Restricted Stock Awards (Details) - 2009 Plan - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Restricted Stock Units (RSUs)      
Number of Shares      
Nonvested, beginning balance (in shares) 505    
Granted (Awarded) (in shares) 245    
Vested (Released) (in shares) (215)    
Forfeited (in shares) (34)    
Nonvested, ending balance (in shares) 501 505  
Weighted-Average Grant Date Fair Value      
Nonvested, beginning balance (in dollars per share) $ 31.42    
Granted (Awarded) (in dollars per share) 45.97 $ 32.58 $ 31.44
Vested (Released) (in dollars per share) 30.41    
Forfeited (in dollars per share) 32.39    
Nonvested, ending balance (in dollars per share) $ 38.90 $ 31.42  
Weighted-Average Remaining Years 1 year 6 months 1 year 7 months 5 days  
Aggregate Intrinsic Value      
Non-vested at December 31, 2017 $ 24,293 $ 17,135  
Fair value of restricted stock units 6,500 4,800 $ 4,700
Unrecognized compensation cost $ 16,100 $ 12,800  
Weighted average period of compensation cost recognized (in years) 2 years 8 months 27 days 2 years 10 months 15 days  
Restricted Stock      
Number of Shares      
Nonvested, beginning balance (in shares) 30    
Granted (Awarded) (in shares) 23    
Vested (Released) (in shares) (30)    
Nonvested, ending balance (in shares) 23 30  
Weighted-Average Grant Date Fair Value      
Nonvested, beginning balance (in dollars per share) $ 31.57    
Granted (Awarded) (in dollars per share) 41.10 $ 31.59 $ 36.05
Vested (Released) (in dollars per share) 31.58    
Nonvested, ending balance (in dollars per share) $ 41.07 $ 31.57  
Aggregate Intrinsic Value      
Fair value of restricted stock units $ 1,000 $ 1,200 $ 1,100
Unrecognized compensation cost $ 300 $ 400  
Weighted average period of compensation cost recognized (in years) 4 months 21 days 4 months 15 days