Shareholders Equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified			0 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jul. 11, 2012 Series K Preferred Stock [Member] Redeemable Preferred Stock [Member] Subsequent Event [Member]	Jul. 11, 2012 Series K Preferred Stock [Member] Public Offering [Member] Redeemable Preferred Stock [Member] Subsequent Event [Member]	Jul. 17, 2012 Series E Preferred Stock [Member] Redeemable Preferred Stock [Member] Subsequent Event [Member]	Aug. 16, 2012 Series E Preferred Stock [Member] Redeemable Preferred Stock [Member] Subsequent Event [Member]
Preferred Stock, Additional Disclosure [Abstract]
Preferred shares of beneficial interest: issued shares	42,184,609	42,186,709		12,000,000
Preferred Stock, Par or Stated Value Per Share				$ 25.00
Preferred Stock, Redemption Price Per Share				$ 25.00
Proceeds from Issuance of Redeemable Preferred Stock				$ 291,923
Preferred Stock, Redemption Amount						$ 75,000
Preferred Stock Issued Additional Disclosure				We retained aggregate net proceeds of $291,923,000, after underwriters’ discounts and issuance costs.
Preferred Stock Duration From Issuance To Earliest Redemption Date				5 years
Preferred Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Preferred Stock, Dividend Rate, Percentage			5.70%		7.00%
Preferred shares of beneficial interest: outstanding shares	42,184,609	42,186,709