Debt (Narrative) (Details) (USD $)	9 Months Ended	12 Months Ended				9 Months Ended		0 Months Ended	9 Months Ended		3 Months Ended	9 Months Ended				3 Months Ended		9 Months Ended		12 Months Ended	9 Months Ended
	Sep. 30, 2011	Dec. 31, 2010	Aug. 29, 2011	Sep. 30, 2011 Minimum [Member] Debt Payable To 2038 [Member]	Sep. 30, 2011 Maximum [Member] Debt Payable To 2038 [Member]	Sep. 30, 2011 Debt Payable To 2038 [Member]	Sep. 30, 2011 Industrial Revenue Bonds Payable To 2015 [Member]	Feb. 11, 2010 Unsecured Revolving Credit Facility [Member]	Sep. 30, 2011 Unsecured Revolving Credit Facility [Member]	Dec. 31, 2010 Unsecured Revolving Credit Facility [Member]	Sep. 30, 2011 7% Unsecured Notes [Member]	Sep. 30, 2011 Unsecured And Uncommitted Overnight Facility [Member]	Dec. 31, 2010 Unsecured And Uncommitted Overnight Facility [Member]	Sep. 30, 2011 Letter Of Credit [Member]	Dec. 31, 2010 Letter Of Credit [Member]	Sep. 30, 2011 3.95 % Convertible Senior Unsecured Notes [Member]	Sep. 30, 2010 3.95 % Convertible Senior Unsecured Notes [Member]	Sep. 30, 2011 3.95 % Convertible Senior Unsecured Notes [Member]	Sep. 30, 2010 3.95 % Convertible Senior Unsecured Notes [Member]	Dec. 31, 2010 3.95 % Convertible Senior Unsecured Notes [Member]	Sep. 30, 2011 Debt Service Guaranty	Dec. 31, 2010 Debt Service Guaranty
Revolving credit facility effective date								February 11, 2010	September 30, 2011			May 2010
Maximum borrowing capacity under credit facility								$ 500,000,000	$ 500,000,000			$ 99,000,000
Credit facility expiration date									September 2015
Debt instrument termination date								September 30, 2011
Credit faciltiy terms description									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					275.0	125.0
Facility fees over LIBOR								50.0	25.0
Bids amount									250,000,000
Term loan			200,000,000
Maximum increase in credit facility amount									700,000,000
Amount outstanding under revolving credit facility									185,000,000	0		10,300,000	80,000,000
Fixed interest rate loan period (days)												30
Letters of credit outstanding														2,300,000	52,400,000
Line of credit facility, interest rate at period end									1.20%			1.50%	1.80%
Available balance under revolving credit facility									312,700,000	447,600,000
Maximum balance outstanding under both facilities	330,700,000	80,000,000
Debt coverage ratio																					1.4
Debt maturity date						2038	2015											2026			2040
Weighted average balance outstanding under both facilities	144,500,000	12,200,000
Weighted average interest rate on credit facilities	1.60%	1.80%
Debt service guaranty liability	74,075,000	97,000,000																			74,100,000	97,000,000
Carrying balance convertible unsecured notes																54,100,000		54,100,000		129,900,000
Debt instrument stated interest percentage				1.50%	8.80%		2.40%				7.00%					3.95%		3.95%
Unamortized discount on convertible unsecured notes																				1,300,000
Effective interest rate of convertible senior unsecured notes																		5.56%		5.75%
Conversion rate																		20.3770
Principal debt value applicable to conversion rate																		$ 1,000
Initial conversion price																$ 49.075		$ 49.075
Net interest expense on convertible notes																1,000,000	2,000,000	5,000,000	6,000,000
Amortization of discount on convertible notes																200,000	500,000	1,300,000	1,700,000
Carrying value of the equity component																23,400,000		23,400,000		23,400,000
Debt Instruments Collateral	1,700,000,000	1,800,000,000
Capital Lease Obligations	45,383,000	21,000,000
Fair value interest rate contracts	11,200,000
Debt premium/(discount) net	3,100,000
Non-cash debt	10,500,000
Principal payments of debt											$ 117,700,000					$ 77,200,000