Purchase Agreement, Controlled Equity Offering and Public Offerings of Common Stock - Additional Information (Detail) - USD ($)							12 Months Ended
	Dec. 16, 2016	Feb. 09, 2016	Jul. 10, 2015	Jun. 11, 2015	Feb. 20, 2015	Apr. 30, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Mar. 03, 2016	Mar. 02, 2016	Apr. 14, 2014	Mar. 31, 2014	Mar. 30, 2014
Stockholders Equity Common Stock [Line Items]
Proceeds from issuance of common stock, net of issuance costs			$ 5,800,000	$ 46,400,000		$ 52,400,000	$ 58,029,000	$ 58,331,000	$ 52,433,000
Common stock, shares issued							115,781,030	100,180,872					48,855,255	244,276,923
Common stock to be sold, shares				16,666,667								12,500,000
Public offering price			$ 3.00	$ 3.00		$ 4.00						$ 4.00
Expected aggregate gross proceeds				$ 50,000,000								$ 50,000,000
Over-allotment option, shares to be issued to underwriters				2,500,000								1,875,000
Shares issued from option exercise by underwriters			2,075,000			1,610,000
Stock issued, price per share			$ 3.00	$ 3.00		$ 4.00						$ 4.00
Additional proceeds from exercise of option by underwriters			$ 6,200,000			$ 6,400,000
Proceeds from issuance of common stock from public offering, net of issuance costs							$ 52,200,000
Issuance costs of Issuance of common shares						4,000,000	4,000,000
Aggregate maximum value of designated securities to be sold						$ 100,000,000	100,000,000			$ 150,000,000	$ 100,000,000
Value of stock to be purchased by existing stockholders												$ 10,000,000
Amount available for future financing of common stock							37,800,000
ATM Offering [Member]
Stockholders Equity Common Stock [Line Items]
Amount available for future financing of common stock							$ 2,400,000
Lincoln Park Capital Fund, LLC [Member]
Stockholders Equity Common Stock [Line Items]
Right to sell common stock	25,000,000
Term to purchase number of shares							36 months
Stock issued, shares							300,000
Additional common stock shares issued on a pro rata basis	152,434
Gross proceeds							$ 412,500,000
Proceeds from issuance of common stock, net of issuance costs							392,500,000
Amount available for future financing of common stock							$ 24,600,000
Cantor Fitzgerald & Co. [Member]
Stockholders Equity Common Stock [Line Items]
Gross proceeds		$ 25,000,000
Proceeds from issuance of common stock, net of issuance costs		$ 24,000,000
Sale of common stock in an at-the-market offering		43,600,000			25,000,000
Percentage of commission paid		3.00%			3.00%
Common stock, shares issued		7,724,488
Sale of common stock in an at-the-market offering		43,600,000			25,000,000
Maximum [Member] | Lincoln Park Capital Fund, LLC [Member]
Stockholders Equity Common Stock [Line Items]
Right to direct to purchase number of shares	150,000
Obligation under regular purchase of shares	$ 2,000,000
Commitment Shares [Member] | Lincoln Park Capital Fund, LLC [Member]
Stockholders Equity Common Stock [Line Items]
Stock issued, shares							312,538