Equity Offering - Additional Information (Detail) - ATM Program - USD ($)	3 Months Ended	9 Months Ended		26 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	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	0	300,227		3,705,936
Average price per share		$ 9.71		$ 13.36