Coal Trading (Details 2) (Coal trading other revenues [Member], USD $) In Millions, unless otherwise specified					3 Months Ended
	Jun. 30, 2013		Dec. 31, 2012		Jun. 30, 2013 External Credit Rating, Investment Grade [Member] Credit Concentration Risk [Member] Coal Trading Positions [Member]	Jun. 30, 2013 External Credit Rating, Non Investment Grade [Member] Credit Concentration Risk [Member] Coal Trading Positions [Member]	Jun. 30, 2013 Non Rated [Member] Credit Concentration Risk [Member] Coal Trading Positions [Member]
Concentration Risk [Line Items]
Credit exposure related to coal trading activities by credit grade, percentage					86.00%	10.00%	4.00%
Potential collateralization that may be requested by counterparties due to a material adverse event	$ 14		$ 8
Margin posted to counterparties related to material adverse event	0		0
Additional potential collateral requirements for a credit downgrade	0		0
Margin posted to counterparties related to credit rating	0		0
Net variation margin held	(126.2)	[1]	(156.9)	[1]
Initial margin posted	20.1		23.2
Margin in excess of the exchange-required variation and initial margin	$ 0.6		$ 0.5

[1] Approximately $88 million and $76 million of the net variation margin held at June 30, 2013 and December 31, 2012, respectively, related to cash flow hedges.