Coal Trading (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Fair value of financial asset (liability) positions measured on a recurring basis
Assets from coal trading activities, Net Basis	$ 52.4		$ 44.6
Liabilities from coal trading activities, Net Basis	(19.4)		(10.3)
Coal Trading [Member]
Trading revenue
Trading revenue	151.8		145.6		158.7
Fair value of financial asset (liability) positions measured on a recurring basis
Assets from coal trading activities, Gross Basis	380.4		170.4
Assets from coal trading activities, Net Basis	52.4		44.6
Liabilities from coal trading activities, Gross Basis	(190.5)		(84.0)
Liabilities from coal trading activities, Net Basis	(19.4)		(10.3)
Subtotal, Gross Basis	189.9		86.4
Subtotal, Net Basis	33.0		34.3
Net variation margin held, Gross Basis	(156.9)	[1]	(52.1)	[1]
Net variation margin held, Net Basis	0	[1]	0	[1]
Net fair value of coal trading positions, Gross Basis	33.0		34.3
Net fair value of coal trading positions, Net Basis	33.0		34.3
Margin held from counterparties and exchanges	156.9		52.1
Commodity swaps and options [Member] | Coal Trading [Member]
Trading revenue
Trading revenue	159.9		(41.4)		23.2
Physical commodity purchase / sale contracts [Member] | Coal Trading [Member]
Trading revenue
Trading revenue	(8.1)		187.0		135.5
Cash Flow Hedging [Member] | Coal Trading [Member]
Fair value of financial asset (liability) positions measured on a recurring basis
Net variation margin held, Net Basis	$ (76.0)		$ (23.0)

[1] Represents margin held from exchanges and counterparties to over-the-counter derivative contracts of $156.9 million and $52.1 million at December 31, 2012 and 2011, respectively. Approximately $76 million and $23 million of the margin held at December 31, 2012 and 2011, respectively, related to cash flow hedges.