Debt (Narrative) (Details) (USD $)				3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011	Aug. 29, 2011	Mar. 31, 2012 Debt Payable To 2038 [Member]	Mar. 31, 2012 Industrial Revenue Bonds Payable To 2015 [Member]	Mar. 31, 2012 Unsecured Revolving Credit Facility [Member]	Mar. 31, 2012 Unsecured And Uncommitted Overnight Facility [Member] D	Mar. 31, 2012 3.95 % Convertible Senior Unsecured Notes [Member]	Mar. 31, 2012 Debt Service Guaranty
Debt Instrument [Line Items]
Revolving credit facility effective date						September 30, 2011	May 2010
Maximum borrowing capacity under credit facility						$ 500,000,000	$ 99,000,000
Credit facility expiration date						The facility expires in September 2015, 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			125.0			125.0
Facility fees over LIBOR						25.0
Bids amount						250,000,000
Unsecured term loan			200,000,000
Maximum increase in credit facility amount						700,000,000
Fixed interest rate loan period (days)							30
Debt coverage ratio									1.4
Debt maturity date				2038	2015			2026	2040
Debt service guaranty liability	74,075,000	74,075,000
Debt instruments collateral	1,700,000,000	1,700,000,000
Unsecured notes payable under revolving credit facilities	170,000,000	166,500,000
Fair value interest rate contracts	10,300,000
Obligations under capital leases	21,000,000	21,000,000
Debt premium/(discount) net	1,600,000
Non-cash debt	$ 10,000,000