Coal Trading (Details 2) (Coal Trading [Member], USD $) In Millions, unless otherwise specified					3 Months Ended
	Mar. 31, 2014		Dec. 31, 2013		Mar. 31, 2014 External Credit Rating, Investment Grade [Member] Credit Concentration Risk [Member] Coal Trading Positions [Member]	Mar. 31, 2014 External Credit Rating, Non Investment Grade [Member] Credit Concentration Risk [Member] Coal Trading Positions [Member]	Mar. 31, 2014 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					83.00%	13.00%	4.00%
Potential collateralization that may be requested by counterparties due to a material adverse event	$ 13		$ 7
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	(51.7)	[1]	(56.1)	[1]
Initial margin posted	16.1		19.6
Margin posted in excess of the exchange-required variation and initial margin	$ 0.4		$ 1.0

[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.