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Summary of Weighted-Average Assumptions of Fair Value Awards (Detail)
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Employee stock purchase plan      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Risk-free interest rate, minimum 1.10% 0.47% 0.26%
Risk-free interest rate, maximum 2.28% 1.10% 0.50%
Expected term 6 months 6 months 6 months
Expected volatility, minimum 49.00% 61.00% 54.00%
Expected volatility, maximum 65.00% 80.00% 99.00%
Employee Stock Option      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Risk-free interest rate, minimum 2.55% 1.70% 1.01%
Risk-free interest rate, maximum 3.03% 2.27% 2.10%
Expected volatility, minimum 59.00% 60.00% 62.00%
Expected volatility, maximum 62.00% 65.00% 66.00%
Employee Stock Option | Minimum      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Expected term 5 years 5 years 5 years
Employee Stock Option | Maximum      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Expected term 6 years 1 month 6 days 6 years 1 month 6 days 6 years 1 month 6 days
Performance Stock Options (PSOs)      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Risk-free interest rate, minimum     1.34%
Risk-free interest rate, maximum     1.50%
Expected volatility, minimum     65.00%
Expected volatility, maximum     66.00%
Performance Stock Options (PSOs) | Minimum      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Expected term     5 years 4 months 24 days
Performance Stock Options (PSOs) | Maximum      
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]      
Expected term     6 years 4 months 24 days