Debt (Narrative) (Details) (USD $)	1 Months Ended	9 Months Ended
	Aug. 29, 2011	Sep. 30, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Medium-term Notes		$ 137,600,000
Debt instrument interest percentage during the period		5.20%
Debt service guaranty liability		74,075,000	74,075,000
Unsecured term loan	200,000,000
Debt maturity date	May 31, 2012
Debt instrument collateral		1,600,000,000	1,700,000,000
Unsecured notes payable under revolving credit facilities		250,000,000	166,500,000
Obligations under capital leases		21,000,000	21,000,000
Fair value of interest rate contracts		10,800,000
Debt premium/(discount) net		2,900,000
Non-cash debt		12,300,000
Unsecured Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Revolving 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
Unsecured And Uncommitted Overnight Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity under credit facility		99,000,000
Credit facility effective date		May 2010
Fixed interest rate loan period (days)		30
3.95% Convertible Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Debt maturity date		2026
Debt instrument stated interest percentage		3.95%
Debt Service Guaranty [Member]
Debt Instrument [Line Items]
Debt Coverage Ratio		1.4
Debt maturity date		2040
Medium-term Notes [Member]
Debt Instrument [Line Items]
Debt maturity date		2022
Medium-term Notes		$ 300,000,000
Debt instrument stated interest percentage		3.38%