Capital Stock - Additional Information (Detail) (USD $)	1 Months Ended	12 Months Ended				1 Months Ended			12 Months Ended
	Mar. 30, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Aug. 16, 2012	Mar. 30, 2012 Underwriters' Overallotment Option	Aug. 31, 2012 7.625% Series A Cumulative Redeemable Preferred Stock	Dec. 31, 2012 2012 Program	Dec. 31, 2012 At The Market Transactions	Dec. 31, 2011 At The Market Transactions	Dec. 31, 2012 At The Market Transactions Common Stock Amount	Dec. 31, 2011 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 Common stock, shares	20,125,000					2,625,000	11,500,000				2,361,500	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		$ 287,500,000	$ 287,500,000				$ 287,500,000
Series A preferred stock, redemption date							Aug. 27, 2017
Issuance of Common stock, offering price	$ 26.81
Issuance of common stock net proceed	539,327,000	587,657,000	860,602,000	271,373,000					66,221,000	48,330,000
Common stock available for sale		200,000,000	200,000,000					10,000,000
Sales commissions and fees											$ 1,037	$ 298