Shareholders' Equity - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Vote $ / shares shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($)	Jul. 28, 2016 USD ($)	Jan. 28, 2015 shares
Stockholders Equity Note [Line Items]
Common stock votes per share | Vote	1
Preferred stock, shares authorized | shares	5,000	5,000
Shares repurchased | shares	0
ATM Program
Stockholders Equity Note [Line Items]
Proceeds from sale of common stock under ATM program	$ 0	$ 45,564,000	$ 0
ATM Program | 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	46,612,000
Proceeds from sale of common stock under ATM program	45,564,000
Commissions paid	$ 1,048,000
ATM Program | Common Stock | Cowen and Company LLC
Stockholders Equity Note [Line Items]
Shares sold under ATM program | shares	3,405,709
Average price per share | $ / shares	$ 13.69
Maximum
Stockholders Equity Note [Line Items]
Board of Directors authorized the repurchase of common stock | shares					2,000,000