Equity (Details Textual) (USD $)	9 Months Ended					9 Months Ended	12 Months Ended	9 Months Ended	12 Months Ended	0 Months Ended	1 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Oct. 25, 2012	Sep. 28, 2012	Dec. 31, 2011	Sep. 30, 2012 7.875% Series A [Member]	Dec. 31, 2011 7.875% Series A [Member]	Sep. 30, 2012 8.00% Series B [Member]	Dec. 31, 2011 8.00% Series B [Member]	Jun. 19, 2012 OvernightProgram [Member]	Oct. 25, 2012 ATMProgram [Member]	Sep. 30, 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	150,000	4,369,087
Common shares issued, net of commissions (per share)										$ 22.10	$ 23.50	$ 23.73
April 2011 Shelf Registration Stmt Maximum	$ 125,000,000
Proceeds from Issuance of Common Stock	218,054,000	235,980,000								109,800,000	3,500,000	102,100,000
Sept 2012 Shelf Registration Stmt Maximu[Line Items]			170,000,000	170,000,000								170,000,000
Prior Shelf Registration Statement Remaining Amount				$ 17,800,000
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