Coal Trading (Details 2) $ in Millions	3 Months Ended
Mar. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Concentration Risk [Line Items]
Number of major credit rating agencies that adjusted corporate credit rating	2
Coal Trading [Member]
Concentration Risk [Line Items]
Potential collateralization that may be requested by counterparties related to material adverse event	$ 36.0	$ 31.0
Margin posted to counterparties related to material adverse event	0.0	0.0
Additional potential collateral requirements for a credit downgrade	0.0	0.0
Margin posted to counterparties related to credit rating	1.0	0.0
Initial margin posted	18.8	15.2
Margin in excess of the exchange-required variation and initial margin	$ 4.2	$ 6.1
External Credit Rating, Investment Grade [Member] \| Credit Concentration Risk [Member] \| Coal Trading Positions [Member] \| Coal Trading [Member]
Concentration Risk [Line Items]
Credit concentration risk percentage	68.00%
External Credit Rating, Non Investment Grade [Member] \| Credit Concentration Risk [Member] \| Coal Trading Positions [Member] \| Coal Trading [Member]
Concentration Risk [Line Items]
Credit concentration risk percentage	8.00%
Non Rated [Member] \| Credit Concentration Risk [Member] \| Coal Trading Positions [Member] \| Coal Trading [Member]
Concentration Risk [Line Items]
Credit concentration risk percentage	24.00%

Additional potential collateral requirements for a credit downgrade.

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Margin posted to counterparties related to credit rating

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Margin posted to counterparties related to material adverse event.

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Number of major credit rating agencies that adjusted the Company's corporate credit rating during the period.

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Potential collateralization that may be requested by counterparties due to a material adverse event.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

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Amount of right to receive cash collateral under master netting arrangements that have not been offset against derivative liabilities.

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Amount receivable from broker-dealers and clearing organizations, including, but not limited to, securities failed-to-deliver, certain deposits for securities borrowed, open transactions, good faith and margin deposits, commissions and floor brokerage receivables.

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