Coal Trading (Details) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Trading revenue
Trading revenue	$ 15.2	$ 52.9	$ 5.3	$ 108.9
Fair value of financial asset (liability) positions measured on a recurring basis
Assets from coal trading activities, Gross Basis	1,095.5		1,095.5		1,706.2
Assets from coal trading activities, Net Basis	109.2		109.2		192.5
Liabilities from coal trading activities, Gross Basis	(1,248.5)		(1,248.5)		(1,843.5)
Liabilities from coal trading activities, Net Basis	(106.1)		(106.1)		(181.7)
Subtotal, Gross Basis	(153.0)		(153.0)		(137.3)
Subtotal, Net Basis	3.1		3.1		10.8
Net margin posted, Gross Basis	156.1		156.1		148.1
Net margin posted, Net Basis	0		0		0
Net value of coal trading positions, Gross Basis	3.1		3.1		10.8
Net value of coal trading positions, Net Basis	3.1		3.1		10.8
Commodity swaps and options [Member]
Trading revenue
Trading revenue	5.8	(36.8)	(26.0)	(9.4)
Physical commodity purchase / sale contracts [Member]
Trading revenue
Trading revenue	$ 9.4	$ 89.7	$ 31.3	$ 118.3