Equity (Details Textual) (USD $)	0 Months Ended	1 Months Ended	0 Months Ended	3 Months Ended					12 Months Ended		3 Months Ended	12 Months Ended			3 Months Ended	12 Months Ended
	Apr. 06, 2011	Jul. 28, 2010	Dec. 14, 2009	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2009	Dec. 31, 2011	Dec. 31, 2010	Mar. 31, 2011 7.875% Series A [Member]	Dec. 31, 2011 7.875% Series A [Member]	Jul. 14, 2011 7.875% Series A [Member]	Mar. 11, 2011 7.875% Series A [Member]	Sep. 30, 2011 8.00% Series B [Member]	Dec. 31, 2011 8.00% Series B [Member]	Sep. 14, 2011 8.00% Series B [Member]
Class of Stock [Line Items]
Preferred shares of beneficial interest, shares issued				9,000,000					9,000,000	0			600,000	5,000,000			3,400,000
Net proceeds, after deducting underwriting discount													$ 15,100,000	$ 120,900,000			$ 82,300,000
Public offering price per share													$ 25.25	$ 25.00			$ 25.00
Percentage of dividend for redeemable preferred shares											7.875%				8.00%
Redemption date of the Company's preferred shares												Mar. 11, 2016				Sep. 21, 2016
Share cap on each preferred shares												$ 2.3234				$ 3.4483
Equity (Textual) [Abstract]
Common shares of beneficial interest, shares authorized				500,000,000					500,000,000	500,000,000
Common shares of beneficial interest, par value				$ 0.01					$ 0.01	$ 0.01
Common shares issued, net of underwriting discounts and offering costs	10,925,000	19,550,000	20,260,000
Common shares raised, net of underwriting discounts and offering costs	$ 226,500,000	$ 318,300,000	$ 379,600,000					$ 379,568,000	$ 443,888,000	$ 318,448,000
Dividend per share/unit				$ 0.12	$ 0.12	$ 0.12	$ 0.12
Preferred shares of beneficial interest, shares authorized				100,000,000					100,000,000	100,000,000
Preferred shares of beneficial interest, par value				$ 0.01					$ 0.01	$ 0.01
Preferred shares of beneficial interest, shares outstanding				9,000,000					9,000,000	0
Preferred Stock, Redemption Price Per Share				$ 25.00					$ 25.00
Preferred stock redemption period									120 days
Operating Partnership outstanding				929,099					929,099	929,099
LTIP units, vested				362,170					362,170	185,820
LTIP units converted to common units				0					0