Purchase Agreement, Controlled Equity Offering and Public Offerings of Common Stock - Schedule of Total Sales under 2015 and 2016 Sales Agreement (Detail) - Cantor Fitzgerald & Co. [Member] - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Feb. 09, 2016	Dec. 31, 2016	Dec. 31, 2015
Sale And Issue Of Shares Under Agreement [Line Items]
Gross proceeds	$ 25,000
2016 Sales Agreement [Member]
Sale And Issue Of Shares Under Agreement [Line Items]
Total shares of common stock sold		8,763,400
Average price per share		$ 4.70
Gross proceeds		$ 41,156
Commissions earned by Cantor		1,235
Other issuance costs		$ 185
2015 Sales Agreement [Member]
Sale And Issue Of Shares Under Agreement [Line Items]
Total shares of common stock sold		5,710,200	2,014,300
Average price per share		$ 3.23	$ 3.25
Gross proceeds		$ 18,454	$ 6,546
Commissions earned by Cantor		$ 553	197
Other issuance costs			$ 259