Notes Payable - Summary of Future Minimum Payments Under Loan Agreement and the Amendments (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	[1]
Debt Instrument [Line Items]
Total notes payable as of March 31, 2019	$ 6,094
Less carrying amount of notes payable	(6,032)	$ (7,396)
Notes payable [Member]
Debt Instrument [Line Items]
2019	4,568
2020	2,231
Total minimum payments	6,799
Less amount representing interest	(705)
Less unamortized debt discount and issuance costs	$ (62)

[1] The condensed consolidated balance sheet as of December 31, 2018, has been derived from the audited financial statements as of that date included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018.