Annual report pursuant to Section 13 and 15(d)

Share-Based Compensation - Activity (Details)

v3.8.0.1
Share-Based Compensation - Activity (Details) - USD ($)
12 Months Ended
Feb. 21, 2017
Feb. 22, 2016
Dec. 31, 2017
Dec. 31, 2016
Unrecognized share-based compensation expense        
Restricted stock awards, Unrecognized compensation expense     $ 10,531,000  
Restricted stock awards, Weighted average remaining vesting period     1 year 3 months 18 days  
Stock options, Unrecognized compensation expense     $ 4,122,000  
Stock options, Weighted average remaining vesting period     1 year 2 months 12 days  
Total unrecognized compensation expense related to unvested awards     $ 14,653,000  
Restricted Stock Awards        
Restricted Stock Awards, Number of Shares        
Balance at beginning of period (in shares)     653,100  
Granted (in shares)     158,900  
Converted/Exercised (in shares)     (187,700)  
Forfeited (in shares)     (33,100)  
Balance at end of period (in shares)     591,200 653,100
Restricted Stock Awards, Weighted Average Grant Date Fair Value Per Share        
Balance at beginning of period (in dollars per share)     $ 25.71  
Granted (in dollars per share)     44.28  
Converted/Exercised (in dollars per share)     24.06  
Forfeited (in dollars per share)     31.08  
Balance at end of period (in dollars per share)     $ 30.92 $ 25.71
Stock Options        
Stock Options, Number of Shares        
Balance at beginning of period (in shares)     712,000  
Granted (in shares)     153,000  
Exercised (in shares)     (181,500)  
Balance at end of period (in shares)     683,500 712,000
Stock Options, Weighted Average Grant Date Fair Value Per Share        
Balance at beginning of period (in dollars per share)     $ 9.73  
Granted (in dollars per share)     14.62  
Converted/Exercised (in dollars per share)     8.56  
Balance at end of period (in dollars per share)     11.13 $ 9.73
Stock Options, Weighted Average Exercise Price Per Share        
Balance at beginning of period (in dollars per share)     25.03  
Granted (in dollars per share)     38.89  
Exercised (in dollars per share)     21.88  
Balance at end of period (in dollars per share)     $ 28.97 $ 25.03
Stock options, additional disclosures        
Aggregate intrinsic value     $ 31,969,700 $ 7,525,800
Converted/Exercised aggregate intrinsic value     $ 6,267,500  
Exercisable, Number of shares     141,400  
Exercisable, Weighted Average Grant Date Fair Value Per Share     $ 10.22  
Exercisable Weighted Average Exercise Price Per Share     $ 26.45  
Exercisable, Aggregate Intrinsic Value     $ 6,970,000  
Weighted average remaining contractual term for vested stock options     7 years 9 months 18 days  
Assumptions used to estimate the fair values of the awards granted:        
Risk free interest rate (as a percent)     2.18% 1.51%
Expected volatility using historical return volatility and implied volatility (as a percent)     35.00% 38.00%
Expected life (in years)     6 years 6 years
Dividend yield (as a percent)     0.00% 0.00%
Weighted-average estimated fair value of options granted during the year (in dollars per share)     $ 14.44 $ 10.20
Market-based conditions | Restricted Stock Awards        
Assumptions used to estimate the fair values of the awards granted:        
Measurement period     2 years 10 months 10 days 2 years 10 months 10 days
Risk free interest rate (as a percent)     1.46% 0.90%
Dividend yield (as a percent)     0.00% 0.00%
Estimated fair value of awards granted (in dollars per share)     $ 50.06 $ 33.77
Performance-based conditions | Restricted Stock Awards        
Restricted stock awards, additional disclosures        
Grant date fair value $ 2,143,000 $ 1,942,000    
Performance-based conditions | Restricted Stock Awards | Payout Range-0%        
Restricted stock awards, additional disclosures        
Payout range (as a percent) 0.00%      
Performance-based conditions | Restricted Stock Awards | Payout Range-25%        
Restricted stock awards, additional disclosures        
Payout range (as a percent) 25.00%      
Related expenses $ 536,000 486,000    
Performance-based conditions | Restricted Stock Awards | Payout Range-100%        
Restricted stock awards, additional disclosures        
Payout range (as a percent) 100.00%      
Related expenses $ 2,143,000 1,942,000    
Performance-based conditions | Restricted Stock Awards | Payout Range-200%        
Restricted stock awards, additional disclosures        
Payout range (as a percent) 200.00%      
Related expenses $ 4,286,000 $ 3,884,000