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Stock-based Compensation - Schedule of ESPP Purchase Rights and Underlying Weighted Average Assumptions (Detail) - ESPP Purchase Rights [Member] - $ / shares
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Weighted average fair value per share $ 9.03 $ 12.03
Minimum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected volatility 59.00% 24.00%
Risk-free interest rate 1.60% 0.70%
Expected term (in years) 6 months 6 months
Maximum [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Expected volatility 107.00% 28.00%
Risk-free interest rate 2.20% 1.20%
Expected term (in years) 2 years 2 years