Equity (Details Textual) (USD $) In Millions, except Share data, unless otherwise specified	0 Months Ended		3 Months Ended		3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended
	Mar. 19, 2012	Mar. 16, 2012	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2012 7.875% Series A [Member]	Dec. 31, 2011 7.875% Series A [Member]	Mar. 31, 2012 8.00% Series B [Member]	Dec. 31, 2011 8.00% Series B [Member]
Class of Stock [Line Items]
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%
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
Common shares of beneficial interest, par value			$ 0.01	$ 0.01
Common shares issued, net of commissions	1,142,803	264,276
Common shares issued, net of commissions (per share)	$ 23.04	$ 23.06
Common shares raised, net of commissions	$ 25.9	$ 6.0
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, redemption price per share			$ 25
Preferred stock redemption period			120 days
Operating Partnership outstanding			929,099	929,099
LTIP units, vested			371,640
LTIP units converted to common units			0