Debt (Narrative) (Details) (USD $)	1 Months Ended	3 Months Ended	12 Months Ended
	Apr. 30, 2003	Mar. 31, 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		1,500,000,000	1,500,000,000
Obligations under capital leases		21,000,000	21,000,000
Fair value of interest rate contracts		8,900,000
Debt premium/(discount), net		(2,500,000)
Non-cash debt		1,500,000
Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Credit facility effective date		Sep. 30, 2011
Maximum borrowing capacity under credit facility		500,000,000
Credit facility expiration date		the facility expires in September 2015 and provides for a one-year extension upon our request and borrowing rates that float at a margin over LIBOR plus a facility fee.
Borrowing margin over LIBOR		125
Facility fees over LIBOR		25
Bids amount		250,000,000
Maximum increase in credit facility amount		700,000,000
Change in borrowing margin over LIBOR		(10)
Change in credit facility fees over LIBOR		(5)
Number of credit facility 6 month extensions		2
Debt maturity date		Apr. 18, 2017
Unsecured And Uncommitted Overnight Facility [Member]
Debt Instrument [Line Items]
Credit facility effective date		May 01, 2010
Maximum borrowing capacity under credit facility		99,000,000
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
Series D Preferred Stock [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%