Coal Trading (Details) (Coal Trading [Member], USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013	Dec. 31, 2013
Principal Transaction Revenue [Line Items]
Trading revenues	$ 21.0		$ 25.9
Coal trading derivative instruments and balance sheet offsetting disclosures:
Assets from coal trading activities, gross amounts of recognized assets	529.6			418.8
Liabilities from coal trading activities, gross amounts of recognized liabilities	(439.3)			(332.7)
Net coal trading assets (liabilities), gross amounts of recognized assets (liabilities)	90.3			86.1
Gross amounts of coal trading liabilities offset against associated coal trading assets	(427.7)			(324.7)
Gross amounts of coal trading assets offset against associated coal trading liabilities	427.7			324.7
Net coal trading (liabilities) assets offset against associated assets (liabilities)	0			0
Variation margin held offset against assets from coal trading activities	(51.7)			(58.0)
Variation margin posted offset against liabilities from coal trading activities	0			1.9
Net variation margin held	(51.7)	[1]		(56.1)	[1]
Assets from coal trading activities, net	50.2			36.1
Liabilities from coal trading activities, net	(11.6)			(6.1)
Net assets from coal trading activities	38.6			30.0
Commodity futures, swaps and options [Member]
Principal Transaction Revenue [Line Items]
Trading revenues	35.6		51.0
Physical commodity purchase / sale contracts [Member]
Principal Transaction Revenue [Line Items]
Trading revenues	(14.6)		(25.1)
Cash Flow Hedging [Member]
Coal trading derivative instruments and balance sheet offsetting disclosures:
Net variation margin held	$ 29.0			$ 42.0

[1] Approximately $29 million and $42 million of the net variation margin held at March 31, 2014 and December 31, 2013, respectively, related to cash flow hedges.