Shareholders Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	9 Months Ended			0 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Jan. 25, 2013 Series L Preferred Stock [Member] Public Offering [Member] Redeemable Preferred Stock [Member]	Feb. 19, 2013 Series F Preferred Stock [Member]	Feb. 19, 2013 Series H Preferred Stock [Member]	Feb. 19, 2013 Series F And Series H Preferred Stock [Member]
Details Of Preferred Shares Of Beneficial Interest [Abstract]
Preferred shares of beneficial interest: issued shares	52,682,807		51,184,609	12,000,000
Preferred Stock Dividend Rate Percentage				5.40%	6.75%	6.75%
Preferred Stock, Par or Stated Value Per Share (in Dollars Per Share)				$ 25.00
Aggregate net proceeds after underwriters discounts and issuance costs				$ 290,536
Purchase Of Outstanding Preferred Units	$ 299,400	$ 243,300					$ 262,500