Common Stock - Additional Information (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015	May. 07, 2015	Mar. 31, 2015	Feb. 28, 2015
Class Of Stock [Line Items]
Common stock sales agreement, aggregate offering amount	$ 58,000	$ 58,000
Offering, issuance and sale of stocks and securities, shelf registration				$ 250,000,000
Common stock, shares issued	58,182,000	58,182,000
Number of additional shares issued	0
Maximum
Class Of Stock [Line Items]
Offering, issuance and sale of stocks and securities, shelf registration			$ 100,000,000
F B R Co
Class Of Stock [Line Items]
Common stock, shares issued	5,900,000
proceeds from sale of shares	$ 10,200,000
Common Stock Available For Sale Under Sales Agreement	$ 9,000,000
F B R Co | Maximum
Class Of Stock [Line Items]
Common stock sales agreement, aggregate offering amount			$ 15,000,000		$ 17,900,000
Common stock sales agreement commission, percentage					3.00%