Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands			9 Months Ended		26 Months Ended
	May 25, 2017	Sep. 15, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Class of Stock [Line Items]
Proceeds from sale of common stock, net			$ 122,046	$ 167,387
Weighted-average remaining lives			2 years 7 days
Common stock
Class of Stock [Line Items]
Common stock issued and sold under sales agreement (in shares)			906,858		5,933,712
Proceeds from sale of common stock, net	$ 72,700	$ 71,000	$ 49,300		$ 146,600
Percentage of gross sales commissions (up to)					3.00%
Stock issued during period (in shares)	1,395,349	2,542,373
Underwriting discounts, fees and expenses	$ 2,300	$ 4,000	$ 600
Price per share issued under underwriting agreement (in dollars per share)	$ 53.75