Stockholders' Equity - Additional Information (Detail) - USD ($)		9 Months Ended
	May 05, 2016	Sep. 30, 2017	Sep. 30, 2016	Aug. 04, 2017
Stockholders Equity Note [Line Items]
Net proceeds from issuance of common stock		$ 18,840,000	$ 22,146,000
Cantor | Sales Agreement | Amendment No. 2
Stockholders Equity Note [Line Items]
Maximum common stock offering price		132,200,000
Common stock registered for sale	$ 70,000,000	$ 42,600,000
Percentage of proceeds payable as compensation to underwriter	2.00%
Common stock, number of shares issued		7,600,000
Net proceeds from issuance of common stock		$ 19,300,000
Cantor | Sales Agreement | Amendment No. 3
Stockholders Equity Note [Line Items]
Common stock registered for sale				$ 70,000,000