Quarterly report pursuant to Section 13 or 15(d)

Share-Based Compensation - Activity (Details)

v3.8.0.1
Share-Based Compensation - Activity (Details) - USD ($)
3 Months Ended
Feb. 19, 2018
Feb. 21, 2017
Feb. 22, 2016
Mar. 31, 2018
Mar. 31, 2017
Dec. 31, 2017
Unrecognized share-based compensation expense            
Restricted stock awards, Unrecognized compensation expense       $ 14,847,000    
Restricted stock awards, Weighted average remaining vesting period       1 year 6 months    
Stock options, Unrecognized compensation expense       $ 5,483,000    
Stock options, Weighted average remaining vesting period       1 year 6 months    
Total unrecognized compensation expense related to unvested awards       $ 20,330,000    
Restricted Stock Awards            
Restricted Stock Awards, Number of Shares            
Balance at beginning of period (in shares)       591,200    
Granted (in shares)       79,000    
Converted/Exercised (in shares)       (106,200)    
Forfeited (in shares)       (20,400)    
Balance at end of period (in shares)       543,600    
Restricted Stock Awards, Weighted Average Grant Date Fair Value Per Share            
Balance at beginning of period (in dollars per share)       $ 30.92    
Granted (in dollars per share)       86.25    
Converted/Exercised (in dollars per share)       23.18    
Forfeited (in dollars per share)       36.06    
Balance at end of period (in dollars per share)       $ 40.28    
Assumptions used to estimate the fair values of the awards granted:            
Measurement period       2 years 10 months 13 days 2 years 10 months 10 days  
Risk free interest rate (as a percent)       2.36% 1.46%  
Dividend yield (as a percent)       0.00% 0.00%  
Estimated fair value of awards granted (in dollars per share)       $ 103.31 $ 50.06  
Stock Options            
Stock Options, Number of Shares            
Balance at beginning of period (in shares)       683,500    
Granted (in shares)       77,300    
Exercised (in shares)       (120,100)    
Forfeited (in shares)       (8,300)    
Balance at end of period (in shares)       632,400    
Stock Options, Weighted Average Grant Date Fair Value Per Share            
Balance at beginning of period (in dollars per share)       $ 11.13    
Granted (in dollars per share)       27.44    
Converted/Exercised (in dollars per share)       10.97    
Forfeited (in dollars per share)       14.44    
Balance at end of period (in dollars per share)       13.11    
Stock Options, Weighted Average Exercise Price Per Share            
Balance at beginning of period (in dollars per share)       28.97    
Granted (in dollars per share)       74.50    
Exercised (in dollars per share)       28.52    
Forfeited (in dollars per share)       38.39    
Balance at end of period (in dollars per share)       $ 34.49    
Stock options, additional disclosures            
Aggregate intrinsic value       $ 26,578,200   $ 31,969,700
Converted/Exercised aggregate intrinsic value       $ 5,528,000    
Exercisable, Number of shares       180,100    
Exercisable, Weighted Average Grant Date Fair Value Per Share       $ 10.59    
Exercisable Weighted Average Exercise Price Per Share       $ 27.37    
Exercisable, Aggregate Intrinsic Value       $ 8,854,600    
Weighted average remaining contractual term for vested stock options       7 years 6 months    
Assumptions used to estimate the fair values of the awards granted:            
Risk free interest rate (as a percent)       2.78% 2.18%  
Expected volatility using historical return volatility and implied volatility (as a percent)       32.50% 35.00%  
Expected life (in years)       6 years 6 years  
Dividend yield (as a percent)       0.00% 0.00%  
Estimated fair value of awards granted (in dollars per share)       $ 27.44 $ 14.44  
Performance-based conditions | Restricted Stock Awards            
Restricted stock awards, additional disclosures            
Grant date fair value $ 2,401,000 $ 2,005,000 $ 1,896,000      
Performance-based conditions | Restricted Stock Awards | Payout Range-0%            
Restricted stock awards, additional disclosures            
Payout range (as a percent) 0.00% 0.00% 0.00%      
Performance-based conditions | Restricted Stock Awards | Payout Range-25%            
Restricted stock awards, additional disclosures            
Payout range (as a percent) 25.00% 25.00% 25.00%      
Related expenses $ 600,000 $ 501,000 $ 474,000      
Performance-based conditions | Restricted Stock Awards | Payout Range-100%            
Restricted stock awards, additional disclosures            
Payout range (as a percent) 100.00% 100.00% 100.00%      
Related expenses $ 2,401,000 $ 2,005,000 $ 1,896,000      
Performance-based conditions | Restricted Stock Awards | Payout Range-200%            
Restricted stock awards, additional disclosures            
Payout range (as a percent) 200.00% 200.00% 200.00%      
Related expenses $ 4,802,000 $ 4,010,000 $ 3,792,000