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Stock Plans - Additional Information (Detail) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value   0 0  
Stock options expiration period       10 years
Stock options exercisable period after grant date   3 years    
Restricted stock and restricted stock unit vest period after grant date   3 years    
Average closing share price, period       1 year
Cash proceeds from exercise of options   $ 8,573 $ 18,465 $ 12,476
Aggregate intrinsic value of stock options exercisable   60,200    
Aggregate intrinsic value of stock options outstanding   69,300    
Total intrinsic value of stock options exercised   12,200 29,300 6,400
Unrecognized stock-based compensation expense   $ 8,100    
Weighted average period over which unrecognized stock-based compensation expense is expected to be recognized, in years   1 year 7 months 28 days    
Minimum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Percentage of target award based on achieving targeted performance conditions   0.00%    
Maximum        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Percentage of target award based on achieving targeted performance conditions   200.00%    
Percentage of target award based on achieving targeted return on net assets based on which final number of shares to be issued for performance stock units   93.60%    
Stock Options        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Excess tax benefit (provision) related to stock plans   $ 2,500 7,400 600
Restricted Stock and Unit Activity        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Fair value of stock units vested   5,400 4,100 3,700
Performance Stock Unit Activity        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Award vesting period (in years) 3 years      
Fair value of stock units vested   $ 1,700 $ 4,100 $ 1,800
Management        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares reserved for future grants   1,054,730    
Non Employee Directors        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares reserved for future grants   114,878    
Stock Options        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Stock options expiration period   10 years