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Assumptions to Estimate Fair Value for Stock Options at Grant Date (Detail) - Time Vested Stock Options
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected life (in years)   4 years
Risk-free interest rate 2.22% 2.54%
Expected volatility 40.00% 53.00%
Expected dividend yield 0.00%  
Minimum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected life (in years) 4 years  
Expected dividend yield   0.00%
Maximum    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected life (in years) 6 years 3 months  
Expected dividend yield   3.57%