Equity (Details Textual) (USD $)	6 Months Ended			6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	0 Months Ended	1 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Jun. 30, 2012 7.875% Series A [Member]	Dec. 31, 2011 7.875% Series A [Member]	Jun. 30, 2012 8.00% Series B [Member]	Dec. 31, 2011 8.00% Series B [Member]	Jun. 19, 2012 OvernightProgram [Member]	Jul. 31, 2012 ATMProgram [Member]	Jun. 30, 2012 ATMProgram [Member]	Jul. 24, 2012 ATMProgram [Member]
Common Stock Disclosures [Abstract]
Common shares of beneficial interest, shares authorized	500,000,000		500,000,000
Common shares of beneficial interest, par value	$ 0.01		$ 0.01
Common shares issued, net of commissions								5,175,000	1,253,228	1,444,527
Common shares issued, net of commissions (per share)								$ 22.10	$ 23.02	$ 23.03
Proceeds from Issuance of Common Stock	$ 147,631,000	$ 235,980,000						$ 109,800,000	$ 28,400,000	$ 32,800,000
Value of common shares available under ATM Program										$ 91,700,000	$ 62,900,000
Preferred Stock Disclosures [Abstract]
Preferred shares of beneficial interest, shares authorized	100,000,000		100,000,000
Preferred shares of beneficial interest, par value	$ 0.01		$ 0.01
Preferred shares of beneficial interest, shares outstanding	9,000,000		9,000,000	5,600,000	5,600,000	3,400,000	3,400,000
Percentage of dividend for redeemable preferred shares				7.875%	7.875%	8.00%	8.00%
Preferred shares of beneficial interest, redemption price per share	$ 25
Share cap on each preferred shares				2.3234		3.4483
LTIP Units Disclosures [Abstract]
Operating Partnership outstanding	929,099		929,099
LTIP units, vested	371,640
LTIP units converted to common units	0