Quarterly report pursuant to Section 13 or 15(d)

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

v3.8.0.1
Stock Based Compensation - Stock Option Activity Under Equity Incentive Plan (Detail) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 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 297,750  
Number of Shares, Exercised (161,705)  
Number of Shares, Forfeitures (24,995)  
Number of Shares, Outstanding, Ending Balance 2,879,761 2,768,711
Number of Shares, Exercisable 1,470,533  
Weighted-Average Exercise Price, Outstanding, Beginning Balance $ 12.07  
Weighted-Average Exercise Price, Granted 10.69  
Weighted-Average Exercise Price, Exercised 0.91  
Weighted-Average Exercise Price, Forfeitures 31.47  
Weighted-Average Exercise Price, Outstanding, Ending Balance 12.38 $ 12.07
Weighted-Average Exercise Price, Exercisable $ 15.89  
Weighted-Average Remaining Contractual Term, Outstanding 6 years 2 months 12 days 5 years 7 months 6 days
Weighted-Average Remaining Contractual Term, Exercisable 2 years 10 months 24 days  
Aggregate Intrinsic Value, Outstanding $ 7.7 $ 11.0
Aggregate Intrinsic Value, Exercisable $ 6.6