v3.8.0.1
Stock-Based Compensation - Narrative (Details)
 1 Months Ended 12 Months Ended
May 31, 2017
director
$ / shares
shares
Dec. 31, 2017
USD ($)
$ / shares
shares
Dec. 31, 2016
USD ($)
$ / shares
shares
Dec. 31, 2015
USD ($)
$ / shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock option vested period   5 years    
Stock option expiration period   10 years    
Options available to be granted | shares   592,300    
Weighted average grant date fair value of options (in usd per share) | $ / shares $ 43.40 $ 12.46 $ 10.64 $ 9.25
Total intrinsic value of options exercised   $ 2,200,000 $ 500,000 $ 700,000
Total unrecognized compensation cost related to non-vested stock options   $ 1,200,000    
Weighted average period of compensation cost   2 years 8 months    
Stock granted to non-employee directors (in shares) | shares 1,000 80,000    
Number of non-employee directors | director 5      
Stock appreciation rights (“SARs”)        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock option vested period   5 years    
Stock option expiration period   10 years    
Weighted average period of compensation cost   3 years 2 months 12 days    
Liability balance   $ 2,500,000 $ 1,200,000  
Stock appreciation rights outstanding (in shares) | shares   366,900 226,500  
Weighted average grant date fair value of SARs (in usd per share) | $ / shares   $ 16.09 $ 13.36 $ 7.33
Intrinsic value of SARs exercised   $ 300,000   $ 0
Total unrecognized compensation cost related to non-vested SARs   $ 3,500,000