Debt (Narrative) (Details) (USD $)	1 Months Ended	6 Months Ended	12 Months Ended
	Apr. 30, 2003	Jun. 30, 2013	Dec. 31, 2012
Debt Instrument [Line Items]
Debt service guaranty liability		$ 74,075,000	$ 74,075,000
Medium-term notes, matured		63,000,000
Debt interest rate during period		5.60%
Debt instrument collateral		1,400,000,000	1,500,000,000
Unsecured notes payable under credit facilities		104,500,000	66,000,000
Obligations under capital leases		21,000,000	21,000,000
Fair value of interest rate contracts		6,800,000
Debt premium/(discount), net		(2,400,000)
Non-cash debt		3,100,000
Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under credit facility		500,000,000
Credit facility effective date		Apr. 18, 2013
Credit facility extension		provides for two consecutive six-month extensions upon our request
Borrowing margin over LIBOR		115
Facility fees over LIBOR		20
Bids amount		250,000,000
Maximum increase in credit facility amount		700,000,000
Number of credit facility 6-month extensions		2
Debt maturity date		Apr. 18, 2017
Unsecured And Uncommitted Overnight Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under credit facility		99,000,000
Credit facility effective date		May 01, 2010
Fixed interest rate loan period (days)		30
Debt Service Guaranty [Member]
Debt Instrument [Line Items]
Debt Coverage Ratio		1.4
Debt maturity date		Jan. 01, 2040
3.5% Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Debt maturity date		Apr. 15, 2023
Senior unsecured notes		300,000,000
Debt stated interest rate		3.50%
Debt issued discount rate		99.53%
Debt effective interest rate		3.56%
Proceeds from issuance of debt		296,600,000
6.75% Series D Preferred Shares [Member]
Debt Instrument [Line Items]
Preferred Shares of Beneficial Interest; redemption value paid		$ 75,000,000
Preferred Shares of Beneficial Interest; dividend percentage	6.75%	6.75%	6.75%