Financial Instruments and Guarantees with Off-Balance-Sheet Risk (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011	Dec. 31, 2010	Sep. 30, 2012	Oct. 31, 2007 T
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	$ 2,271.6
Cash up front from sale of receivables	2,834.5
Financial Instruments and Guarantees with Off-Balance-Sheet Risk (Textuals) [Abstract]
Letters of Credit Outstanding, Amount	79.7
Amount in bank guarantees, letters of credit and surety bonds related to collateral for surety companies, road maintenance, performance guarantees and other operations	185.7
DTA Lease Term	30
Maximum capacity of the securitization program	275.0
Amount available under the securitization program	191.5
Total consideration received by Company related to accounts receivable sold under securitization program	4,377.8
Additional cash upon collection of underlying receivables	1,247.7
Non collected receivables	295.6
Reduction in accounts receivable as a result of securitization	25.0		150.0
Expense associated with securitization transactions	2.0		2.0	2.4
Number of company operated mines included in Patriot Spin-off						8
Number of majority owned JV mines included in the Patriot spin-off						2
Number of coal preparation facilities servicing contractor-operatedines included in the Patriot spin-off						8
Proven and probable coal reserves related to Patriot						1,200,000,000
Potential exposure from Patriot bankruptcy					150
Patriot collateral posted with the Department of Labor			15
Dominion Terminal Associates Partnership [Member]
Financial Instruments and Guarantees with Off-Balance-Sheet Risk (Textuals) [Abstract]
Equity Method Investment, Ownership Percentage	37.50%
Number of Letters of Credit Supporting Reimbursement Obligation	4
Maximum reimbursement obligation to commercial bank related to Dominion Terminals	42.7
Pension Plans Agreement With PBG and TXU Europe [Member]
Financial Instruments and Guarantees with Off-Balance-Sheet Risk (Textuals) [Abstract]
Letter of credit maintained by the Company in favor of the PBGC	37.0
Guarantee in place from TXU Europe Limited	110.0
Reclamation Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	1,847.4
Lease Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	105.3
Workers' Compensation Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	53.5
Guarantee Type, Other [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	265.4	[1]
Self bonding [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	1,275.8
Self bonding [Member] | Reclamation Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	1,275.8
Self bonding [Member] | Lease Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	0
Self bonding [Member] | Workers' Compensation Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	0
Self bonding [Member] | Guarantee Type, Other [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	0	[1]
Surety bonds [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	460.0
Surety bonds [Member] | Reclamation Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	316.5
Surety bonds [Member] | Lease Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	105.3
Surety bonds [Member] | Workers' Compensation Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	29.4
Surety bonds [Member] | Guarantee Type, Other [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	8.8	[1]
Bank guarantees [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	430.4
Bank guarantees [Member] | Reclamation Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	255.1
Bank guarantees [Member] | Lease Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	0
Bank guarantees [Member] | Workers' Compensation Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	0
Bank guarantees [Member] | Guarantee Type, Other [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	175.3	[1]
Letters of credit [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	105.4
Letters of credit [Member] | Reclamation Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	0
Letters of credit [Member] | Lease Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	0
Letters of credit [Member] | Workers' Compensation Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	24.1
Letters of credit [Member] | Guarantee Type, Other [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	$ 81.3	[1]

[1] Other includes the $79.7 million in letters of credit described below and an additional $185.7 million in bank guarantees, letters of credit and surety bonds related to collateral for surety companies, road maintenance, performance guarantees and other operations.