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Share-Based Compensation - Summary of Share-Based Compensation Expense by Award Type (Detail) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based compensation expense $ 24,916 $ 17,597 $ 44,314 $ 30,864
Options        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based compensation expense 204 836 739 1,964
Restricted stock units        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based compensation expense 23,669 16,014 41,633 27,418
Employee stock purchase plan rights        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based compensation expense $ 1,043 $ 747 $ 1,942 $ 1,482