Debt (Narrative) (Details) (USD $)	3 Months Ended			6 Months Ended		12 Months Ended
	Sep. 30, 2011	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Unsecured notes payable under revolving credit facilities		$ 177,850,000		$ 177,850,000		$ 80,000,000
Maximum balance outstanding under both facilities				180,000,000		80,000,000
Weighted average balance outstanding under both facilities				100,500,000		12,200,000
Weighted average interest rate on credit facilities		1.70%		1.70%		1.80%
Debt service guaranty liability		74,075,000		74,075,000		97,000,000
Debt Instruments Collateral		1,800,000,000		1,800,000,000		1,800,000,000
Capital Lease Obligations		45,383,000		45,383,000		21,000,000
Fair value interest rate contracts		7,600,000		7,600,000
Debt premium/(discount) net		3,700,000		3,700,000
Non-cash debt		10,900,000		10,900,000
Term loan	200,000,000
Minimum [Member] | Debt Payable To 2038 [Member]
Debt instrument stated interest percentage		2.60%		2.60%
Maximum [Member] | Debt Payable To 2038 [Member]
Debt instrument stated interest percentage		8.80%		8.80%
Debt Payable To 2038 [Member]
Debt maturity date				2038
Industrial Revenue Bonds Payable To 2015 [Member]
Debt maturity date				2015
Debt instrument stated interest percentage		2.40%		2.40%
Unsecured Revolving Credit Facility [Member]
Revolving credit facility effective date				February 11, 2010
Maximum borrowing capacity under credit facility		500,000,000		500,000,000
Credit facility expiration date				February 2013
Borrowing margin over LIBOR		275.0		275.0
Facility fees over LIBOR		50.0		50.0
Bids amount		250,000,000		250,000,000
Maximum increase in credit facility amount		700,000,000		700,000,000
Amount outstanding under revolving credit facility		165,000,000		165,000,000		0
Line of credit facility, interest rate at period end		1.30%		1.30%
Available balance under revolving credit facility		332,700,000		332,700,000		447,600,000
7% Unsecured Notes [Member]
Debt instrument stated interest percentage	7.00%
Principal payments of debt	117,700,000
Unsecured and Uncommitted Overnight Facility [Member]
Revolving credit facility effective date				May 2010
Maximum borrowing capacity under credit facility		99,000,000		99,000,000
Amount outstanding under revolving credit facility		12,900,000		12,900,000		80,000,000
Fixed interest rate loan period (days)		30		30
Line of credit facility, interest rate at period end		1.60%		1.60%		1.80%
Letter of Credit [Member]
Letters of credit outstanding		2,300,000		2,300,000		52,400,000
3.95 % Convertible Senior Unsecured Notes [Member]
Debt maturity date				2026
Carrying balance convertible unsecured notes		131,100,000		131,100,000		129,900,000
Debt instrument stated interest percentage		3.95%		3.95%
Unamortized discount on convertible unsecured notes		200,000		200,000		1,300,000
Effective interest rate of convertible senior unsecured notes				5.75%		5.75%
Conversion rate				20.3770
Principle debt value applicable to conversion rate				$ 1,000
Initial conversion price		$ 49.075		$ 49.075
Net interest expense on convertible notes		2,000,000	2,000,000	4,000,000	4,000,000
Amortization of discount on convertible notes		600,000	500,000	1,100,000	1,100,000
Carrying value of the equity component		23,400,000		23,400,000		23,400,000
Principal payments of debt	77,200,000
Debt Service Guaranty
Debt coverage ratio		1.4		1.4
Debt maturity date				2040
Debt service guaranty liability		$ 74,100,000		$ 74,100,000		$ 97,000,000