Financial Instruments and Guarantees with Off-Balance-Sheet Risk (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Reclamation, coal lease and workers' compensation obligations
Reclamation Obligations	$ 1,690.0		$ 1,690.0
Lease Obligations	110.3		110.3
Workers' Compensation Obligations	68.9		68.9
Other	244.6		244.6
Total	2,113.8		2,113.8
Financial Instruments and Guarantees with Off-Balance-Sheet Risk (Textuals) [Abstract]
Amount in letters of credit and surety bonds related to collateral for surety companies, road maintenance, performance guarantees and other operations	162.1		162.1
Interest in a partnership			37.50%
Time period of lease that permits the partnership to purchase the terminal, in years			30 years
Maximum reimbursement obligation to commercial bank related to Dominion Terminals	42.7		42.7
Letter of credit maintained by the Company in favor of the PBGC	37.0		37.0
Number of benefit pension plans	2		2
Guarantee in place from TXU Europe Limited			110.0
Letter of credit for collateral for bank	2.8		2.8
Maximum capacity of the securitization program	275.0		275.0
Amount available under the securitization program	4.5		4.5
Total consideration received by Company related to accounts receivable sold under securitization program			2,376.8
Cash up front from sale of receivables			1,805.9
Additional cash upon collection of underlying receivables			344.8
Non collected receivables	226.1		226.1
Reduction in accounts receivable as a result of securitization activity			150.0		150.0
Expense associated with securitization transactions	0.5	0.5	1.1	1.2
The Company's undivided interest percentage in the Prairie State Energy Campus			5.06%
Self bonding [Member]
Reclamation, coal lease and workers' compensation obligations
Reclamation Obligations	936.6		936.6
Lease Obligations	0		0
Workers' Compensation Obligations	0		0
Other	0		0
Total	936.6		936.6
Surety bonds [Member]
Reclamation, coal lease and workers' compensation obligations
Reclamation Obligations	613.5		613.5
Lease Obligations	110.3		110.3
Workers' Compensation Obligations	6.2		6.2
Other	10.8		10.8
Total	740.8		740.8
Bank guarantees [Member]
Reclamation, coal lease and workers' compensation obligations
Reclamation Obligations	139.9		139.9
Lease Obligations	0		0
Workers' Compensation Obligations	0		0
Other	151.3		151.3
Total	291.2		291.2
Letters of credit [Member]
Reclamation, coal lease and workers' compensation obligations
Reclamation Obligations	0		0
Lease Obligations	0		0
Workers' Compensation Obligations	62.7		62.7
Other	2.8		2.8
Total	65.5		65.5
Bilateral cash collateralization agreements [Member]
Reclamation, coal lease and workers' compensation obligations
Reclamation Obligations	0		0
Lease Obligations	0		0
Workers' Compensation Obligations	0		0
Other	79.7		79.7
Total	$ 79.7		$ 79.7