Financial Instruments and Guarantees with Off-Balance-Sheet Risk (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended			9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011	Sep. 30, 2012		Sep. 30, 2011	Dec. 31, 2011	Oct. 31, 2007 T
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	$ 2,295.1			$ 2,295.1
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	216.0			216.0
Time Period Of Lease That Permits The Partnership To Purchase The Terminal				30 years
Maximum capacity of the securitization program	275.0			275.0
Amount available under the securitization program	153.1			153.1
Total consideration received by Company related to accounts receivable sold under securitization program				3,210.2
Cash up front from sale of receivables				1,973.7
Additional cash upon collection of underlying receivables				874.6
Non collected receivables	362.0			362.0
Reduction in accounts receivable as a result of securitization	100.0			100.0			150.0
Expense associated with securitization transactions	0.5		0.4	1.4		1.5
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 (tons)								1,200,000,000
Potential exposure from Patriot bankruptcy	150			150
Patriot collateral posted with the Department of Labor							15
DTA and PBGC [Member]
Guarantee Obligations [Line Items]
Letters of Credit Outstanding, Amount	79.7			79.7
Dominion Terminal Associates Partnership [Member]
Financial Instruments and Guarantees with Off-Balance-Sheet Risk (Textuals) [Abstract]
Equity Method Investment, Ownership Percentage	37.50%			37.50%
Number of Letters of Credit Supporting Reimbursement Obligation				4
Maximum reimbursement obligation to commercial bank related to Dominion Terminals	42.7			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			37.0
Number of benefit pension plans	2			2
Guarantee in place from TXU Europe Limited	110.0			110.0
Reclamation Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	1,819.9			1,819.9
Property Lease Guarantee [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	105.3			105.3
Workers' Compensation Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	74.2			74.2
Guarantee Type, Other [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	295.7	[1]		295.7	[1]
Self bonding [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	1,221.9			1,221.9
Self bonding [Member] | Reclamation Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	1,221.9			1,221.9
Self bonding [Member] | Property Lease Guarantee [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	0			0
Self bonding [Member] | Workers' Compensation Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	0			0
Self bonding [Member] | Guarantee Type, Other [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	0	[1]		0	[1]
Surety bonds [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	499.0			499.0
Surety bonds [Member] | Reclamation Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	372.1			372.1
Surety bonds [Member] | Property Lease Guarantee [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	105.3			105.3
Surety bonds [Member] | Workers' Compensation Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	11.8			11.8
Surety bonds [Member] | Guarantee Type, Other [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	9.8	[1]		9.8	[1]
Bank guarantees [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	430.6			430.6
Bank guarantees [Member] | Reclamation Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	225.9			225.9
Bank guarantees [Member] | Property Lease Guarantee [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	0			0
Bank guarantees [Member] | Workers' Compensation Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	0			0
Bank guarantees [Member] | Guarantee Type, Other [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	204.7	[1]		204.7	[1]
Letters of credit [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	143.6			143.6
Letters of credit [Member] | Reclamation Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	0			0
Letters of credit [Member] | Property Lease Guarantee [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	0			0
Letters of credit [Member] | Workers' Compensation Obligations [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	62.4			62.4
Letters of credit [Member] | Guarantee Type, Other [Member]
Guarantee Obligations [Line Items]
Guarantor Obligations, Current Carrying Value	$ 81.2	[1]		$ 81.2	[1]

[1] Other includes the $79.7 million in letters of credit described below and an additional $216.0 million in bank guarantees, surety bonds and letters of credit related to collateral for surety companies, road maintenance, performance guarantees and other operations.