Equity (Details Textual) (USD $)	9 Months Ended			9 Months Ended		9 Months Ended		9 Months Ended		0 Months Ended	9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013	Sep. 30, 2014 6.50% Series C [Member]	Dec. 31, 2013 6.50% Series C [Member]	Sep. 30, 2014 7.875% Series A [Member]	Dec. 31, 2013 7.875% Series A [Member]	Sep. 30, 2014 8.00% Series B [Member]	Dec. 31, 2013 8.00% Series B [Member]	Sep. 09, 2014 Sept 9 common offering [Member]	Sep. 30, 2014 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
Stock Issued During Period, Shares, New Issues										3,450,000	400,000
Stock Issued During Period, Value, New Issues	$ 170,750,000	$ 101,242,000								$ 38.15	$ 38.09
Mar 2014 Shelf Registration Stmt Maximum Amount											175,000,000
Proceeds from Issuance of Common Stock	146,854,000	4,829,000								131,600,000	15,000,000
Sept 2012 Shelf Registration Stmt Maximum Amount											170,000,000
Amount Available under ATM Program											159,800,000
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 Stock, Shares Issued	14,000,000		13,000,000	1,000,000
Preferred Stock, Value, Issued	140,000		130,000	25.00
Proceeds from Issuance of Preferred Stock and Preference Stock	$ 25,000,000	$ 100,000,000		$ 24,500,000
Preferred Stock, Shares Outstanding	14,000,000		13,000,000	5,000,000	4,000,000	5,600,000	5,600,000	3,400,000	3,400,000
Preferred Stock, Dividend Rate, Percentage				6.50%		7.875%		8.00%
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	607,991		607,991
LTIP units, vested	195,290