Annual report pursuant to Section 13 and 15(d)

Stockholders' Equity - Additional Information (Details)

v3.20.4
Stockholders' Equity - Additional Information (Details)
$ / shares in Units, $ in Millions
12 Months Ended
Jun. 20, 2013
shares
Dec. 31, 2020
USD ($)
director
$ / shares
shares
Dec. 31, 2019
USD ($)
$ / shares
shares
Dec. 31, 2018
USD ($)
$ / shares
shares
Dec. 31, 2006
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Percentage of directors' stock grant vested on one year anniversary date   100.00%      
Granted (in dollars per share) | $ / shares   $ 123.89 $ 110.56 $ 64.11  
Total unrecognized compensation cost related to nonvested shares of common stock granted | $   $ 30.7      
Weighted average period of cost recognized   2 years 11 months 1 day      
Fair value of shares vested | $   $ 12.2 $ 9.4 $ 8.0  
Compensation expense related to nonvested stock grants | $   $ 14.3 $ 9.6 $ 9.4  
Price of shares issued under employee stock purchase plan as a percentage of FMV         95.00%
Number of shares reserved for the employee stock purchase plan (in shares)         250,000
Additional shares authorized for issuance (in shares) 250,000        
Shares redeemed to satisfy personal tax obligations (in shares)   78,767 107,461 138,925  
Reclassification of equity for exercised stock options | $   $ 0.9      
Employee          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Period of vested shares   5 years      
Nonvested stock grants to employees (in shares)   114,680 163,250 213,105  
Consultant          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Nonvested stock grants to employees (in shares)   10,890      
Directors          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Nonvested stock grants to Independent directors (in shares)   9,900 17,880 13,600  
Retired Director          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Nonvested stock grants to Independent directors (in shares)   775      
Number of retired directors granted shares | director   1      
Percentage vested on grant date   100.00%      
2018 Long-Term Incentive Plan          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Common stock reserved and available for issuance (in shares)       2,210,544  
Shares available for issuance (in shares)       1,873,199  
Nonvested stock          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Aggregate intrinsic value, outstanding | $   $ 13.2      
Aggregate intrinsic value, exercised | $   $ 2.4