Purchase Agreement, Controlled Equity Offering and Public Offering of Common Stock - Schedule of Total Sales under 2017 Sales Agreement (Detail) - Stifel, Nicolaus & Company, Incorporated [Member] - 2017 Sales Agreement [Member] $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Sale And Issue Of Shares Under Agreement [Line Items]
Total shares of common stock sold | shares	15,998,500
Average price per share | $ / shares	$ 3.13
Gross proceeds	$ 50,000
Commissions earned by Stifel	1,500
Other issuance costs	$ 97