Equity (Details Textual) (USD $) In Millions, except Share data, unless otherwise specified			0 Months Ended		3 Months Ended		12 Months Ended	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 18, 2013 6.50% Series C [Member]	Mar. 19, 2012 6.50% Series C [Member]	Mar. 31, 2013 6.50% Series C [Member]	Mar. 31, 2013 7.875% Series A [Member]	Dec. 31, 2012 7.875% Series A [Member]	Mar. 31, 2013 8.00% Series B [Member]	Dec. 31, 2012 8.00% Series B [Member]	Mar. 31, 2013 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
Sept 2012 Shelf Registration Stmt Maximum Amount										$ 170.0
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
Issuance of shares, net of offering costs, Shares			3,600,000
Preferred Stock, Dividend Rate, Percentage					6.50%	7.875%	7.875%	8.00%	8.00%
Preferred shares issued, net of commissions (per share)			$ 25.00
Preferred Stock, Shares Outstanding	12,600,000	9,000,000			3,600,000	5,600,000	5,600,000	3,400,000	3,400,000
Preferred shares of beneficial interest, redemption price per share	$ 25.00
Share cap on each preferred shares					2.0325	2.3234		3.4483
LTIP Units Disclosures [Abstract]
Operating Partnership outstanding	381,109	381,109
LTIP units, vested	9,470
Net proceeds from issuance of preferred shares				$ 87.1