Purchase Agreement, Controlled Equity Offering and Public Offering of Common Stock - Schedule of Total Sales under 2015, 2016 and 2017 Sales Agreement (Detail) - USD ($) $ / shares in Units, $ in Thousands		12 Months Ended
	Feb. 09, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Stifel, Nicolaus & Company, Incorporated [Member] | 2017 Sales Agreement [Member]
Sale And Issue Of Shares Under Agreement [Line Items]
Total shares of common stock sold		15,998,500
Average price per share		$ 3.13
Gross proceeds		$ 50,000
Commissions earned by Cantor		1,500
Other issuance costs		$ 97
Cantor Fitzgerald & Co. [Member]
Sale And Issue Of Shares Under Agreement [Line Items]
Gross proceeds	$ 25,000
Cantor Fitzgerald & Co. [Member] | 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
Cantor Fitzgerald & Co. [Member] | 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