Share and Unit Information - Additional Information (Detail) (USD $)	3 Months Ended			6 Months Ended							3 Months Ended		6 Months Ended
	Jun. 30, 2011	Mar. 31, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Aug. 26, 2010	Nov. 05, 2009	Jul. 03, 2008	Nov. 03, 2006 Maximum	Jun. 30, 2011 At The Market (ATM) Programs	Jun. 30, 2010 At The Market (ATM) Programs	Jun. 30, 2011 At The Market (ATM) Programs	Jun. 30, 2010 At The Market (ATM) Programs
Stockholders Equity Note [Line Items]
Common stock, shares outstanding	37,142,477		32,299,493	37,142,477	32,299,493	34,871,399
Operating partnership unit outstanding	1,951,819		2,198,090	1,951,819	2,198,090
Total common shares and operating partnership units outstanding	39,094,296		34,497,583	39,094,296	34,497,583
Common stock shares, outstanding option	14,457		21,557	14,457	21,557
Agreement to sell common stock with Financial Service companies, shares							6,000,000	4,000,000	1,350,000	2,000,000
Common stock, shares issued											528,674	2,503,600	1,552,473	3,074,600
Proceeds from Issuance of common stock				$ 125,737,000	$ 161,999,000						$ 34,100,000	$ 131,600,000	$ 95,300,000	$ 161,500,000
Common stock issued, Dividend and Distribution Reinvestment and Share Purchase Program	151		187	495,387	521
Proceeds from Issuance of common stock, Dividend and Distribution Reinvestment and Share Purchase Program	$ 10,200		$ 10,300	$ 30,000,000	$ 27,500
Common stock acquired to satisfy tax withholding obligation that arose upon vesting of restricted stock granted		25,082