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Stock-Based Compensation - ESPP (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Sep. 30, 2016
Weighted-average assumptions used to estimate the fair value of ESPP shares        
Estimated dividend yield 0.00%      
Stock-based compensation expense $ 21,317 $ 5,056 $ 374  
ESPP        
Stock-Based Compensation        
Shares remained available for grant 500      
Maximum annual increase in shares available for grant (in shares) 800,000      
Maximum employee payroll deduction (as a percent) 100.00%      
Maximum offering period 2 years      
Period between purchases 6 months      
Price of ESPP shares as percentage of market price 85.00%      
Weighted-average assumptions used to estimate the fair value of ESPP shares        
Expected term (years) 10 months 24 days 9 months 18 days    
Volatility (as a percent) 45.90% 48.90%    
Risk-free interest rate (as a percent) 1.22% 0.69%    
Estimated dividend yield 0.00% 0.00%    
Holding period for purchases after the first offering period   6 months    
Holding period for purchases during the first offering period   12 months    
Stock-based compensation expense $ 10,700 $ 3,300    
Unrecognized employee stock-based compensation $ 6,600      
Unrecognized employee stock-based compensation, recognition period 8 months 12 days      
ESPP | First Offering Period        
Weighted-average assumptions used to estimate the fair value of ESPP shares        
Stock-based compensation expense $ 10,100 $ 3,300    
ESPP | Common Stock        
Stock-Based Compensation        
Maximum annual increase in shares available for issuance, percentage of outstanding shares 1.00%      
ESPP | Class A common stock        
Stock-Based Compensation        
Stock available for issuance (in shares)       800,000