Stockholders' Equity - Additional Information (Detail) - USD ($)		1 Months Ended	9 Months Ended
	Jan. 08, 2018	Jan. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	May 05, 2016
Stockholders Equity Note [Line Items]
Proceeds from common stock sold			$ 79,301,000	$ 18,840,000
Amendment No. 2 | Cantor | Sales Agreement
Stockholders Equity Note [Line Items]
Maximum common stock offering price			132,200,000
Common stock registered for sale					$ 70,000,000
Amendment No. 3 | Cantor | Sales Agreement
Stockholders Equity Note [Line Items]
Proceeds from common stock sold			22,300,000
Maximum common stock offering price	$ 70,000,000
Common stock registered for sale			$ 47,300,000
Percentage of proceeds payable as compensation to underwriter	2.00%
Unsold shares of common stock, value	$ 31,400,000
Common stock, number of shares issued			3,200,000
Common Stock | Underwritten Public Offering
Stockholders Equity Note [Line Items]
Common stock, shares issued and sold		14,030,000
Common stock, par value		$ 0.001
Common stock, sale of stock, price per share		$ 4.10
Proceeds from common stock sold		$ 57,200,000