Quarterly report pursuant to Section 13 or 15(d)

Stock Based Compensation - Stock Option Activity Under Equity Incentive Plan (Detail)

v3.10.0.1
Stock Based Compensation - Stock Option Activity Under Equity Incentive Plan (Detail) - USD ($)
$ / shares in Units, $ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]    
Number of Shares, Outstanding, Beginning Balance 2,768,711  
Number of Shares, Granted 1,783,287  
Number of Shares, Exercised (167,786)  
Number of Shares, Forfeitures (182,770)  
Number of Shares, Outstanding, Ending Balance 4,201,442 2,768,711
Number of Shares, Exercisable 1,556,112  
Weighted-Average Exercise Price, Outstanding, Beginning Balance $ 12.07  
Weighted-Average Exercise Price, Granted 7.05  
Weighted-Average Exercise Price, Exercised 0.94  
Weighted-Average Exercise Price, Forfeitures 20.40  
Weighted-Average Exercise Price, Outstanding, Ending Balance 10.02 $ 12.07
Weighted-Average Exercise Price, Exercisable $ 14.36  
Weighted-Average Remaining Contractual Term, Outstanding 7 years 1 month 6 days 5 years 7 months 6 days
Weighted-Average Remaining Contractual Term, Exercisable 3 years 1 month 6 days  
Aggregate Intrinsic Value, Outstanding $ 3.4 $ 11.0
Aggregate Intrinsic Value, Exercisable $ 3.3