Debt (Narrative) (Details) (USD $)	1 Months Ended				6 Months Ended	12 Months Ended	6 Months Ended	12 Months Ended	6 Months Ended
	May 31, 2012	Aug. 29, 2011	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2012 Debt Payable To 2038 [Member]	Dec. 31, 2011 Debt Payable To 2038 [Member]	Jun. 30, 2012 Industrial Revenue Bonds Payable To 2015 [Member]	Dec. 31, 2011 Industrial Revenue Bonds Payable To 2015 [Member]	Jun. 30, 2012 Unsecured Revolving Credit Facility [Member]	Jun. 30, 2012 Unsecured And Uncommitted Overnight Facility [Member] D	Jun. 30, 2012 3.95% Convertible Senior Unsecured Notes [Member]	Jun. 30, 2012 Debt Service Guaranty [Member]
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 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									125.0
Facility fees over LIBOR									25.0
Bids amount									250,000,000
Maximum increase in credit facility amount									700,000,000
Fixed interest rate loan period (days)										30
Debt Coverage Ratio												1.4
Debt maturity date	May 2012	May 31, 2012			2038	2038	2015	2015			2026	2040
Debt service guaranty liability			74,075,000	74,075,000
Unsecured term loan		200,000,000
Fixed-rate debt	115,000,000
Debt instrument stated interest percentage	5.30%						2.40%	2.40%			3.95%
Debt instruments collateral			1,600,000,000	1,700,000,000
Unsecured notes payable under revolving credit facilities			229,000,000	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			1,600,000
Non-cash debt			$ 9,600,000