Debt (Narrative) (Details) (USD $)	3 Months Ended	12 Months Ended	3 Months Ended			3 Months Ended		1 Months Ended			1 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Unsecured Revolving Credit Facility [Member] extensions	Oct. 31, 2013 Unsecured Revolving Credit Facility [Member]	Mar. 31, 2013 Unsecured Revolving Credit Facility [Member]	Mar. 31, 2014 Unsecured And Uncommitted Overnight Facility [Member]	Mar. 31, 2014 Debt Service Guaranty [Member]	Oct. 31, 2013 4.45% Unsecured Senior Notes [Member]	Mar. 31, 2013 3.5% Senior Unsecured Notes [Member]	Dec. 31, 2013 6% Fixed-Rate Note [Member]	Mar. 31, 2013 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	$ 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 maturity date							Jan. 01, 2040	Jan. 15, 2024	Apr. 15, 2023
Debt service guaranty liability	73,740,000	73,740,000
Senior unsecured notes								250,000,000	300,000,000
Debt stated interest rate								4.45%	3.50%	6.00%
Debt issued at discount								99.58%	99.53%
Debt effective interest rate								4.50%	3.56%
Proceeds from issuance of debt								247,300,000	296,600,000
Preferred shares of beneficial interest; redemption value paid											75,000,000
Preferred shares of beneficial interest; dividend percentage											6.75%
Medium term notes, matured	315,000,000	173,600,000
Debt interest rate during period	5.20%	5.40%
Debt payable	1,841,427,000	2,205,104,000								100,000,000
Debt instruments collateral value	$ 1,100,000,000	$ 1,200,000,000