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Stock Incentive Plans - Summary of Stock Option Activity Under 2006 and 2016 Plans, Including Grants to Non-employees (Details) - USD ($)
$ / shares in Units, $ in Thousands
 9 Months Ended 12 Months Ended
Sep. 30, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]      
Options Outstanding, Beginning balance 2,977,218 2,685,913  
Options granted 449,556 361,385  
Options exercised (649,213) (57,067)  
Options forfeited (11,659) (13,013)  
Options Outstanding, Ending balance 2,765,902 2,977,218 2,685,913
Options exercisable 1,656,418    
Options vested and expected to vest 2,698,510    
Options Outstanding, Weighted Average Exercise Price Per Share, Beginning Balance $ 6.16 $ 4.81  
Options Outstanding, Weighted Average Exercise Price Per Share, Options granted 36.24 15.65  
Options Outstanding, Weighted Average Exercise Price Per Share, Options exercised 3.97 2.33  
Options Outstanding, Weighted Average Exercise Price Per Share, Options forfeited 15.72 6.92  
Options Outstanding, Weighted Average Exercise Price Per Share, Ending Balance 11.52 $ 6.16 $ 4.81
Options Outstanding, Weighted Average Exercise Price Per Share, Exercisable 5.43    
Options Outstanding, Weighted Average Exercise Price Per Share, vested and expected to vest $ 11.21    
Options Outstanding, Weighted- Average Remaining Contractual Life (years) 7 years 3 months 15 days 6 years 11 months 5 days 7 years 7 months 17 days
Options exercisable, Weighted- Average Remaining Contractual Life (years) 6 years 4 months 10 days    
Options vested and expected to vest, Weighted- Average Remaining Contractual Life (years) 7 years 3 months    
Options Outstanding, Aggregate Intrinsic Value $ 111,634 $ 70,979 $ 11,589
Options exercisable, Aggregate Intrinsic Value 76,940    
Options vested and expected to vest, Aggregate Intrinsic Value $ 109,750