Equity Offering - Additional Information (Detail) - ATM Program - USD ($)	3 Months Ended	6 Months Ended		35 Months Ended
	Jun. 30, 2019	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jul. 28, 2016
Stockholders Equity Note [Line Items]
Proceeds from sale of common stock under ATM program		$ 4,606,000	$ 2,849,000
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	$ 1,259,000	4,713,000		$ 54,240,000
Proceeds from sale of common stock under ATM program	1,231,000	4,607,000		53,019,000
Commissions paid	$ 28,000	$ 106,000		$ 1,220,000
Common Stock | Cowen and Company LLC
Stockholders Equity Note [Line Items]
Shares sold under ATM program	356,350	1,250,000		4,955,936
Average price per share	$ 3.53	$ 3.77		$ 10.94