Purchase Agreement, Controlled Equity Offering and Public Offering of Common Stock - Additional Information (Detail) - USD ($)							4 Months Ended	12 Months Ended
	Aug. 31, 2017	Apr. 28, 2017	Feb. 09, 2016	Jul. 10, 2015	Jun. 11, 2015	Feb. 20, 2015	Apr. 27, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 16, 2016	Mar. 03, 2016	Mar. 02, 2016
Stockholders Equity Common Stock [Line Items]
Warrants to purchase common shares		24,900,000
Offering price		$ 1.00		$ 3.00	$ 3.00
Gross proceeds from public offering		$ 24,900,000			$ 50,000,000
Net proceeds from public offering		$ 23,200,000			$ 46,400,000
Warrants exercised		0
Proceeds from issuance of common stock, net of issuance costs				$ 5,800,000
Common stock to be sold, shares					16,666,667
Over-allotment option, shares to be issued to underwriters					2,500,000
Shares issued from option exercise by underwriters				2,075,000
Additional proceeds from exercise of option by underwriters				$ 6,200,000
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
Aggregate maximum value of designated securities to be sold								$ 100,000,000			$ 150,000,000	$ 100,000,000
Common stock, shares issued								199,282,003	115,781,030
Lincoln Park Capital Fund, LLC [Member]
Stockholders Equity Common Stock [Line Items]
Right to sell common stock										25,000,000
Stock issued, shares								3,972,741	300,000
Gross proceeds								$ 5,304,000	$ 412,500
Proceeds from issuance of common stock, net of issuance costs								$ 5,304,000	$ 392,500
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
Percentage of commission paid			3.00%			3.00%
Common stock, shares issued			7,724,488
Cantor Fitzgerald & Co. [Member] | Maximum [Member]
Stockholders Equity Common Stock [Line Items]
Sale of common stock in an at-the-market offering			$ 43,600,000			$ 25,000,000
Stifel, Nicolaus & Company, Incorporated [Member]
Stockholders Equity Common Stock [Line Items]
Percentage of commission paid	3.00%
Stifel, Nicolaus & Company, Incorporated [Member] | Maximum [Member]
Stockholders Equity Common Stock [Line Items]
Sale of common stock in an at-the-market offering	$ 50,000,000
Commitment Shares [Member] | Lincoln Park Capital Fund, LLC [Member]
Stockholders Equity Common Stock [Line Items]
Stock issued, shares							345,421
Series B Warrant [Member]
Stockholders Equity Common Stock [Line Items]
Warrants exercised								8,893,700
Common Stock [Member]
Stockholders Equity Common Stock [Line Items]
Number of common stock or warrants in each unit		1
Common Stock [Member] | Series A Warrant [Member]
Stockholders Equity Common Stock [Line Items]
Number of common stock or warrants in each unit		1
Common Stock [Member] | Series B Warrant [Member]
Stockholders Equity Common Stock [Line Items]
Number of common stock or warrants in each unit		0.75