Stockholders' Equity - Additional Information (Detail) - USD ($) shares in Millions	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	May 05, 2016
Stockholders Equity Note [Line Items]
Proceeds from common stock sold	$ 22,146,000	$ 75,327,000
Cantor | Sales Agreement
Stockholders Equity Note [Line Items]
Maximum common stock offering price	132,200,000
Common stock registered for sale	$ 62,300,000		$ 70,000,000
Percentage of proceeds payable as compensation to underwriter	2.00%
Common stock, number of shares issued	3.5
Proceeds from common stock sold	$ 22,000,000