Coal Trading (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
Fair value of financial asset (liability) positions measured on a recurring basis
Assets from coal trading activities, net	$ 56.2			$ 56.2			$ 44.6
Liabilities from coal trading activities, Net Basis	(19.9)			(19.9)			(10.3)
Coal Trading [Member]
Fair value of financial asset (liability) positions measured on a recurring basis
Assets from coal trading activities, Gross Basis	444.8			444.8			170.4
Assets from coal trading activities, net	56.2			56.2			44.6
Liabilities from coal trading activities, Gross Basis	(216.6)			(216.6)			(84.0)
Liabilities from coal trading activities, Net Basis	(19.9)			(19.9)			(10.3)
Subtotal, Gross Basis	228.2			228.2			86.4
Subtotal, Net Basis	36.3			36.3			34.3
Net margin held, Gross Basis	191.9	[1]		191.9	[1]		52.1	[1]
Net margin held, Net Basis	0	[1]		0	[1]		0	[1]
Net value of coal trading positions, Gross Basis	36.3			36.3			34.3
Net value of coal trading positions, Net Basis	36.3			36.3			34.3
Trading revenue
Trading revenue	40.3		29.0	132.8		34.3
Commodity swaps and options [Member] | Coal Trading [Member]
Trading revenue
Trading revenue	60.3		(0.6)	110.9		(26.6)
Physical commodity purchase / sale contracts [Member] | Coal Trading [Member]
Trading revenue
Trading revenue	$ (20.0)		$ 29.6	$ 21.9		$ 60.9

[1] (1) Represents margin held from exchanges of $191.9 million and $52.1 million at September 30, 2012 and December 31, 2011, respectively. Approximately $100 million and $23 million of the margin held at September 30, 2012 and December 31, 2011, respectively, related to cash flow hedges.