Stockholders' Equity - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Millions	1 Months Ended	6 Months Ended
	Apr. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	May 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Net proceeds from sale of shares, after deducting underwriting discounts and commissions and other estimated offering expenses		$ 215,756,000	$ 81,562,000
ATM Agreement [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock issuable and sellable under offering agreement		$ 75,000,000
ATM Agreement [Member] | Maximum [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Common stock issuable and sellable under offering agreement				$ 75,000,000
Common Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Sale of common stock in underwritten public offering	14.2
Sale of common stock, public offering price	$ 16.25
Net proceeds from sale of shares, after deducting underwriting discounts and commissions and other estimated offering expenses	$ 215,800,000