Coal Trading (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended					6 Months Ended
	Jul. 31, 2014 credit_rating_notch	Jun. 30, 2014 Coal Trading [Member]		Dec. 31, 2013 Coal Trading [Member]		Jun. 30, 2014 External Credit Rating, Investment Grade [Member] Credit Concentration Risk [Member] Coal Trading Positions [Member] Coal Trading [Member]	Jun. 30, 2014 External Credit Rating, Non Investment Grade [Member] Credit Concentration Risk [Member] Coal Trading Positions [Member] Coal Trading [Member]	Jun. 30, 2014 Non Rated [Member] Credit Concentration Risk [Member] Coal Trading Positions [Member] Coal Trading [Member]
Concentration Risk [Line Items]
Credit exposure related to coal trading activities by credit grade, percentage						94.00%	2.00%	4.00%
Potential collateralization that may be requested by counterparties due to a material adverse event		$ 15		$ 7
Margin posted to counterparties related to material adverse event		0		0
Number of major credit rating agencies that adjusted corporate credit rating	2
Total number of major credit rating agencies in period that rating was adjusted	3
Severity of credit rating downgrade	1
Additional potential collateral requirements for a credit downgrade		0		0
Margin posted to counterparties related to credit rating		0		0
Net variation margin held		(34.0)	[1]	(56.1)	[1]
Initial margin posted		17.1		19.6
Margin posted in excess of the exchange-required variation and initial margin		$ 2.0		$ 1.0

[1] Approximately $15 million and $42 million of the net variation margin held at June 30, 2014 and December 31, 2013, respectively, related to cash flow hedges.