Shareholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			9 Months Ended
	Jul. 30, 2018	Jul. 12, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Apr. 28, 2017
Class of Warrant or Right [Line Items]
Issuance of common stock upon at the market offerings, net			$ 493	$ 991
Common stock, par value			$ 0.001		$ 0.001
Value of shares sold prior to amendment			$ 45,792
Common and preferred stock, shares authorized			201,000,000
Common stock, shares authorized			200,000,000		500,000,000
Preferred stock, shares authorized			1,000,000
Preferred stock, par value			$ 0.001
Common Stock [Member]
Class of Warrant or Right [Line Items]
Common stock, shares issued			4,250,000
Value of shares sold prior to amendment			$ 4
Underwriters Public Offering [Member]
Class of Warrant or Right [Line Items]
Issuance of common stock upon at the market offerings, net	$ 45,800
Underwriters Public Offering [Member] | Common Stock [Member]
Class of Warrant or Right [Line Items]
Common stock, shares issued	4,250,000
Share price	$ 11.50
H.C. Wainwright & Co., LLC [Member]
Class of Warrant or Right [Line Items]
Common stock, par value						$ 0.001
Sales commissions in fixed percentage of gross proceeds per share						3.00%
Common stock, shares sold			46,900
Net proceeds from sale of shares			$ 500
H.C. Wainwright & Co., LLC [Member] | Common Stock [Member]
Class of Warrant or Right [Line Items]
Value of shares sold prior to amendment		$ 2,300
H.C. Wainwright & Co., LLC [Member] | Maximum [Member]
Class of Warrant or Right [Line Items]
Amount available to sell under equity program						$ 60,000