Stockholders' Equity - Additional Information (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	0 Months Ended		12 Months Ended
	Nov. 14, 2013	Nov. 01, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Stockholders' Equity [Line Items]
Public offering, common stock		4,693,876
Common stock per share price		$ 12.25
Net proceeds from common stock offerings	$ 54,434		$ 12,857	$ 62,092	$ 8,895
Net proceeds from preferred stock offerings	$ 106,882			$ 106,882
Series E [Member]
Stockholders' Equity [Line Items]
Public offering, depositary shares		4,000,000
Depositary shares, price per share		$ 25.00
Common Stock Over-Allotment Option [Member]
Stockholders' Equity [Line Items]
Public offering, common stock		612,244
Preferred Stock Over-Allotment Option [Member]
Stockholders' Equity [Line Items]
Public offering, depositary shares	435,080