Preferred Stock and Capital Stock - Additional Information (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	9 Months Ended				1 Months Ended		3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Aug. 16, 2012	Dec. 31, 2011	Aug. 31, 2012 7.625% Series A Cumulative Redeemable Preferred Stock	Sep. 30, 2012 2012 Program	Sep. 30, 2012 At The Market Transactions	Sep. 30, 2012 At The Market Transactions	Sep. 30, 2012 At The Market Transactions Common Stock Amount	Sep. 30, 2012 At The Market Transactions Common Stock Amount
Class of Stock [Line Items]
Increased number of authorized preferred stock	25,000,000		25,000,000	25,000,000
Authorized preferred stock before amendment			10,000,000
Preferred stock, par value	$ 0.001		$ 0.001	$ 0.001	$ 0.001
Issuance of stock, shares					11,500,000				595,000	1,713,900
Shares issued pursuant to full exercise of underwriters' over allotment option					1,500,000
Liquidation preference per share					$ 25.00
Series A preferred stock dividend rate per year					7.625%
Annual dividend per share on the preferred stock					$ 1.90625
Total liquidation preference value	$ 287,500			$ 287,500	$ 287,500
Series A preferred stock, redemption date					Aug. 27, 2017
Common stock available for sale	200,000,000			200,000,000		10,000,000
Issuance of common stock net proceeds	587,677	853,441					17,087	48,357
Sales commissions and fees									$ 268	$ 810