Shareholder's Equity - Additional Information (Detail) (USD $)	12 Months Ended			0 Months Ended		1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Nov. 13, 2013	Mar. 18, 2013	Aug. 31, 2014
Stockholders Equity Note [Line Items]
Proceeds from issuance of common stock	$ 170,372,000	$ 274,295,000	$ 111,468,000
Shares available for issuance	120,000,000	120,000,000
Common Stock
Stockholders Equity Note [Line Items]
Number of public offerings		2
Underwritten Public Offering | Common Stock
Stockholders Equity Note [Line Items]
Issuance of stocks (in shares)				4,500,000	8,050,000	6,900,000
Issuance of shares, offer price (in dollars per share)				$ 15.90	$ 15.55	$ 16.44
Over-allotment shares purchased by underwriters						900,000
Proceeds from issuance of common stock				70,400,000	122,200,000	108,700,000
Controlled Equity Offering | Common Stock
Stockholders Equity Note [Line Items]
Issuance of stocks (in shares)	3,800,000	5,400,000
Issuance of shares, offer price (in dollars per share)	$ 16.50	$ 15.10
Proceeds from issuance of common stock	61,700,000	81,700,000
Sales commissions and fees on issuance of shares	$ 900,000	$ 1,200,000
Shares available for issuance	4,000,000.0	8,000,000.0