Debt (Narrative) (Details) (USD $)	12 Months Ended		12 Months Ended				12 Months Ended		1 Months Ended			1 Months Ended		1 Months Ended	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2013 Unsecured Revolving Credit Facility [Member] extensions	Oct. 31, 2013 Unsecured Revolving Credit Facility [Member]	Mar. 31, 2013 Unsecured Revolving Credit Facility [Member]	Oct. 31, 2012 Unsecured Revolving Credit Facility [Member]	Dec. 31, 2013 Unsecured And Uncommitted Overnight Facility [Member]	Dec. 31, 2013 Debt Service Guaranty [Member]	Oct. 31, 2013 4.45% Senior Unsecured Notes [Member]	Mar. 31, 2013 3.5% Senior Unsecured Notes [Member]	Dec. 31, 2013 6% Fixed Rate Note [Member]	Oct. 31, 2012 3.38% Senior Unsecured Notes [Member]	Oct. 31, 2012 3.95% Convertible Senior Unsecured Notes [Member]	Mar. 31, 2013 6.75% Series D Preferred Shares [Member]	Dec. 31, 2013 6.75% Series D Preferred Shares [Member]	Dec. 31, 2012 6.75% Series D Preferred Shares [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under credit facility			$ 500,000,000	$ 500,000,000	$ 500,000,000	$ 500,000,000	$ 99,000,000
Credit facility effective date			Apr. 18, 2013				May 01, 2010
Credit facility expiration date			Apr. 18, 2017
Credit facility description of extensions			provides for two consecutive six-month extensions upon our request
Number of credit facility 6-month extensions			2
Borrowing margin over LIBOR, basis points			115
Facility fees, basis points			20
Bids amount			250,000,000
Maximum increase in credit facility amount			700,000,000
Fixed interest rate loan period (in days)							30 days
Debt coverage ratio								1.4
Debt service guaranty liability	73,740,000	74,075,000
Senior unsecured notes									250,000,000	300,000,000		300,000,000
Debt stated interest rate									4.45%	3.50%	6.00%	3.38%	3.95%
Debt maturity date								Jan. 01, 2040	Jan. 15, 2024	Apr. 15, 2023		Oct. 15, 2022
Debt issued at discount									99.58%	99.53%		99.62%
Debt effective interest rate									4.50%	3.56%		3.42%
Proceeds from issuance of debt									247,300,000	296,600,000		296,900,000
Preferred Shares of beneficial interest, redemption value paid														75,000,000	75,000,000
Preferred shares of beneficial interest, dividend percentage														6.75%	6.75%	6.75%
Medium-term notes, matured	173,600,000
Debt interest rate during period	5.40%
Debt payable											100,000,000
Convertible senior unsecured debt													54,100,000
Debt instruments collateral value	$ 1,200,000,000	$ 1,500,000,000