Equity Offering - Additional Information (Detail) - ATM Program - USD ($)	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Jun. 30, 2017	Jul. 28, 2016
Stockholders Equity Note [Line Items]
Proceeds from sale of common stock under ATM program		$ 2,849,000	$ 0
Cowen and Company LLC
Stockholders Equity Note [Line Items]
Aggregate offering price authorized				$ 75,000,000
Proceeds from sale of common stock under ATM program gross	$ 2,914,000	49,527,000
Proceeds from sale of common stock under ATM program	2,849,000	48,412,000
Commissions paid	$ 65,000	$ 1,114,000
Common Stock | Cowen and Company LLC
Stockholders Equity Note [Line Items]
Shares sold under ATM program	300,227	3,705,936
Average price per share	$ 9.71	$ 13.36