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Stock-Based Compensation - Additional Information (Details) - USD ($)
1 Months Ended 12 Months Ended
Oct. 31, 2015
Jun. 30, 2012
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Jan. 01, 2019
Jan. 01, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Shares reserved for issuance, shares     5,549,908 4,277,531      
Aggregate intrinsic value of options exercised     $ 21,100,000 $ 9,800,000 $ 337,000    
Estimated grant date fair value of options vested     13,400,000 4,800,000 2,100,000    
Unamortized stock-based compensation     $ 45,900,000 $ 16,900,000 $ 7,500,000    
Stock option average expected recognition period     3 years 1 month 6 days 1 year 8 months 12 days 2 years 8 months 12 days    
Unrecognized share based compensation expense     $ 180,000 $ 174,000 $ 180,000    
Weighted average grant date fair value of options granted     $ 34.70 $ 9.92 $ 7.59    
Unvested common shares outstanding     9,790 81,373      
Right of repurchase of shares, lapse rate     25.00%        
Percentage of shares of original grant per month for 36 months thereafter     2.083%        
Accrued liabilities and other long-term liabilities associated with shares issued subject to repurchase rights     $ 15,000 $ 68,000      
Unvested Early Exercised Stock              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Shares repurchased     997 41,961      
Payments for repurchase of shares     $ 1,000 $ 45,000      
Employee Stock Purchase Plan              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Number of shares reserved for issuance under the option plan, description     An annual increase on January 1 of each year beginning on January 1, 2017        
Stock Options              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Shares reserved for issuance, shares     3,864,407 2,964,549      
RSU              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Total grant date fair value vested     $ 159,000    
Stock option average expected recognition period     3 years 2 months 12 days        
Unamortized stock-based compensation relating to RSUs     $ 7,000,000        
2012 Equity Incentive Plan | Incentive Stock Options | Minimum              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Percentage of estimated fair value of common stock   100.00%          
2012 Equity Incentive Plan | Nonstatutory Stock Options | Minimum              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Percentage of estimated fair value of common stock   85.00%          
2012 Equity Incentive Plan | Stock Options              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Terms of options granted     10 years        
Vesting period     4 years        
2012 Equity Incentive Plan | Stock Options | Minimum              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Percentage of estimated fair value of common stock   110.00%          
2012 Equity Incentive Plan | Stock Options | Maximum              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Employee owning voting rights of stock options   10.00%          
2015 Stock Option and Incentive Plan              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Vesting period     4 years        
Shares reserved for issuance, shares     904,785 863,538     1,432,511
Number of shares reserved for issuance under the option plan, description     (i) the number of shares represented by awards outstanding under the Company’s 2012 Equity Incentive Plan that are forfeited or lapse unexercised and which following the pricing date are not issued under the 2012 Plan, and (ii) an annual increase on January 1 of each year beginning on January 1, 2017.        
2015 Stock Option and Incentive Plan | Subsequent Event              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Shares reserved for issuance, shares           1,611,557  
2015 Stock Option and Incentive Plan | IPO              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Shares reserved for issuance, shares 1,650,000            
2015 Equity Incentive Plan | Employee Stock Purchase Plan              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Percentage of estimated fair value of common stock 85.00%            
Shares reserved for issuance, shares 255,000           358,127
Number of common stock purchased by one employee 2,500            
2015 Equity Incentive Plan | Employee Stock Purchase Plan | Subsequent Event              
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]              
Shares reserved for issuance, shares           402,889