Equity Line Financing, Controlled Equity Offering and Public Offering of Common Stock - Schedule of Total Sales under 2015 and 2016 Sales Agreement (Detail) - Cantor Fitzgerald & Co. [Member] - USD ($)		6 Months Ended	12 Months Ended
	Feb. 09, 2016	Jun. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Sale And Issue Of Shares Under Agreement [Line Items]
Gross proceeds	$ 25,000,000	$ 0
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			$ 41,156,000
Commissions earned by Cantor			1,235,000
Other issuance costs			$ 185,000
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			$ 18,454,000	$ 6,546,000
Commissions earned by Cantor			$ 553,000	197,000
Other issuance costs				$ 259,000