Subsequent Events Subsequent Events (Narrative) (Details) (USD $)	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Subsequent Event [Line Items]
Medium-term Notes	$ 137,600,000
Medium-term Notes [Member]
Subsequent Event [Line Items]
Medium-term Notes	300,000,000
Debt Instrument, Interest Rate, Stated Percentage	3.38%
Debt maturity date	2022
Debt Instrument, Interest Rate, Effective Percentage	3.42%
Debt Instrument, Unamortized Discount Rate	99.62%
Proceeds from Issuance of Long-term Debt	296,900,000
3.95% Convertible Senior Unsecured Notes [Member]
Subsequent Event [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.95%
Debt maturity date	2026
Convertible senior unsecured notes	54,100,000
Debt Redemption Date	Nov. 08, 2012
Unsecured Revolving Credit Facility [Member]
Subsequent Event [Line Items]
Maximum borrowing capacity under credit facility	500,000,000
6.95% Series E Preferred Shares [Member]
Subsequent Event [Line Items]
Preferred Shares of Beneficial Interest; dividend percentage	6.95%	6.95%
Preferred Shares of Beneficial Interest; liquidation preference	$ 72,500,000	$ 72,500,000
Preferred Shares of Beneficial Interest; redemption date	Nov. 21, 2012