Stockholders' Equity - Additional Information (Detail) - USD ($) $ in Thousands, shares in Millions		6 Months Ended
	Jan. 23, 2018	Jun. 30, 2018	Jun. 30, 2017
Class of Stock [Line Items]
Proceeds from sale of common stock, net		$ 135,972	$ 122,046
Common stock
Class of Stock [Line Items]
Common stock issued and sold under sales agreement (in shares)		1.0
Proceeds from sale of common stock, net		$ 62,300
Underwriting discounts, fees and expenses	$ 900	$ 500
Stock issued during period (in shares)	1.3
Proceeds from issuance of common stock, net of underwriting discounts, fees and expenses	$ 74,100