Purchase Agreement, Controlled Equity Offering and Public Offering of Common Stock - Additional Information (Detail) - USD ($)						4 Months Ended	12 Months Ended
	Dec. 28, 2018	Aug. 31, 2017	Apr. 28, 2017	Feb. 09, 2016	Feb. 20, 2015	Apr. 27, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 16, 2016
Stockholders Equity Common Stock [Line Items]
Warrants to purchase common shares			24,900,000
Offering price			$ 1.00
Gross proceeds from public offering			$ 24,900,000
Net proceeds from public offering			$ 23,200,000
Warrants exercised			0
Series B Warrant [Member]
Stockholders Equity Common Stock [Line Items]
Warrants exercised							8,893,700	7,052,215
Stifel, Nicolaus & Company, Incorporated [Member]
Stockholders Equity Common Stock [Line Items]
Percentage of commission paid	3.00%	3.00%
Total shares of common stock sold							0
Stifel, Nicolaus & Company, Incorporated [Member] | Maximum [Member]
Stockholders Equity Common Stock [Line Items]
Sale of common stock in an at-the-market offering	$ 75,000,000	$ 50,000,000
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
Lincoln Park Capital Fund, LLC [Member] | Commitment Shares [Member]
Stockholders Equity Common Stock [Line Items]
Stock issued, shares						345,421
Cantor Fitzgerald & Co. [Member]
Stockholders Equity Common Stock [Line Items]
Percentage of commission paid				3.00%	3.00%
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
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