Equity (Details Textual) (USD $)	0 Months Ended	12 Months Ended			0 Months Ended
	Dec. 31, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2014	Oct. 30, 2014	Sep. 09, 2014
Common Stock Disclosures [Abstract]
Common shares of beneficial interest, shares authorized	500,000,000	500,000,000	500,000,000
Common shares of beneficial interest, par value	$ 0.01	$ 0.01	$ 0.01
Stock Issued During Period, Value, New Issues		$ 316,910,000	$ 175,575,000	$ 214,871,000
Proceeds from Issuance of Common Stock		293,211,000	79,362,000	221,579,000
Preferred Stock Disclosures [Abstract]
Preferred shares of beneficial interest, shares authorized	100,000,000	100,000,000	100,000,000
Preferred shares of beneficial interest, par value	$ 0.01	$ 0.01	$ 0.01
Preferred Stock, Shares Issued	14,000,000	14,000,000	13,000,000
Preferred Stock, Value, Issued	140,000	140,000	130,000
Proceeds from Issuance of Preferred Stock and Preference Stock		25,000,000	100,000,000	0
Preferred Stock, Shares Outstanding	14,000,000	14,000,000	13,000,000
Preferred shares of beneficial interest, redemption price per share	$ 25.00	$ 25.00
LTIP Units Disclosures [Abstract]
Operating Partnership outstanding	236,351	236,351	607,991
LTIP units, vested	0	0
Long Term Incentive Partnership Units Converted To Common Units		258,101	0	444,535
Long Term Incentive Partnership Units Converted to Cash	113,539
Payments for Repurchase of Redeemable Noncontrolling Interest	5,200,000	5,231,000	0	2,342,000
6.50% Series C [Member]
Preferred Stock Disclosures [Abstract]
Preferred Stock, Shares Issued					1,000,000
Preferred Stock, Value, Issued	25.00	25.00
Proceeds from Issuance of Preferred Stock and Preference Stock					24,500,000
Preferred Stock, Shares Outstanding	5,000,000	5,000,000	4,000,000
Preferred Stock, Dividend Rate, Percentage		6.50%
Share cap on each preferred shares		2.0325
7.875% Series A [Member]
Preferred Stock Disclosures [Abstract]
Preferred Stock, Shares Outstanding	5,600,000	5,600,000	5,600,000
Preferred Stock, Dividend Rate, Percentage		7.875%
Share cap on each preferred shares		2.3234
8.00% Series B [Member]
Preferred Stock Disclosures [Abstract]
Preferred Stock, Shares Outstanding	3,400,000	3,400,000	3,400,000
Preferred Stock, Dividend Rate, Percentage		8.00%
Share cap on each preferred shares		3.4483
Sept 9 common offering [Member]
Common Stock Disclosures [Abstract]
Stock Issued During Period, Shares, New Issues						3,680,000
Stock Issued During Period, Value, New Issues							38.15
Proceeds from Issuance of Common Stock							131,600,000
ATMProgram [Member]
Common Stock Disclosures [Abstract]
Stock Issued During Period, Shares, New Issues		400,000
Stock Issued During Period, Value, New Issues		38.09
Mar 2014 Shelf Registration Stmt Maximum Amount	175,000,000	175,000,000
Proceeds from Issuance of Common Stock		15,000,000
Sept 2012 Shelf Registration Stmt Maximum Amount	170,000,000	170,000,000
Amount Available under ATM Program	159,800,000	159,800,000
Oct 30 common offering [Member]
Common Stock Disclosures [Abstract]
Stock Issued During Period, Shares, New Issues							3,450,000
Stock Issued During Period, Value, New Issues						39.77
Proceeds from Issuance of Common Stock						146,400,000