Coal Trading (Details) (USD $) In Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2011		Sep. 30, 2010	Sep. 30, 2011		Sep. 30, 2010	Dec. 31, 2010
Fair value of financial asset (liability) positions measured on a recurring basis
Assets from coal trading activities, Net Basis	$ 66.0			$ 66.0			$ 192.5
Liabilities from coal trading activities, Net Basis	(62.6)			(62.6)			(181.7)
Coal Trading [Member] | Commodity swaps and options [Member]
Trading revenue
Trading revenue	(0.6)		38.9	(26.6)		29.5
Coal Trading [Member] | Physical commodity purchase / sale contracts [Member]
Trading revenue
Trading revenue	29.6		23.2	60.9		141.5
Coal Trading [Member]
Trading revenue
Trading revenue	29.0		62.1	34.3		171.0
Fair value of financial asset (liability) positions measured on a recurring basis
Assets from coal trading activities, Gross Basis	590.5			590.5			1,706.2
Assets from coal trading activities, Net Basis	66.0			66.0			192.5
Liabilities from coal trading activities, Gross Basis	(660.5)			(660.5)			(1,843.5)
Liabilities from coal trading activities, Net Basis	(62.6)			(62.6)			(181.7)
Subtotal, Gross Basis	(70.0)			(70.0)			(137.3)
Subtotal, Net Basis	3.4			3.4			10.8
Net margin posted, Gross Basis	(73.4)	[1]		(73.4)	[1]		(148.1)	[1]
Net margin posted, Net Basis	0	[1]		0	[1]		0	[1]
Net value of coal trading positions, Gross Basis	3.4			3.4			10.8
Net value of coal trading positions, Net Basis	$ 3.4			$ 3.4			$ 10.8

[1] Represents margin posted with counterparties and exchanges of $73.4 million at September 30, 2011; and margin posted with counterparties and exchanges of $148.2 million, net of margin held of $0.1 million, at December 31, 2010. In addition, at December 31, 2010, the Company held letters of credit of $5.0 million from counterparties in lieu of margin posted. Of the margin posted at September 30, 2011, approximately 94% related to cash flow hedges.