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Stock Compensation Plans - Additional Information (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Stock options      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Fair value of option granted $ 6.4 $ 6.0 $ 6.3
Fair value of option vested 5.2 $ 7.8 $ 9.5
Unrecognized compensation cost related to unvested stock $ 5.5    
Recognized over a remaining weighted-average vesting period (year) 1 year 5 months 12 days    
Stock options | Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Options expiration term from the grant date (in years) 10 years    
Stock options | Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period (in years) 3 years    
Restricted Stock Units (RSUs)      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Unrecognized compensation cost related to unvested stock $ 1.9    
Recognized over a remaining weighted-average vesting period (year) 1 year 3 months 21 days    
Restricted Stock Units (RSUs) | Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period (in years) 3 years    
Stock Options and Restricted Stock Units (RSUs) | Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Common stock authorized for issuance 46,700,000    
Shares available for future grants 14,000,000