Future Minimum Payments Under Loans Payable (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt Disclosure [Abstract]
2018 (remaining 6 months)	$ 985
2019	8,755
2020	9,041
2021	6,104
Long-term Debt, Gross, Total	24,885
Less amount representing interest	(3,795)
Less unamortized discount	(1,270)	$ (1,500)
Less deferred charges	(1,090)
Less loans payable current, net of discount	(2,388)
Loans payable, net of current portion and discount	$ 16,342	$ 18,477