Coal Trading (Details 2) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Dec. 31, 2014
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	$ 31
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	1	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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- Definition

Margin posted to counterparties related to credit rating

+ References

No definition available.

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Margin posted to counterparties related to material adverse event.

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- Definition

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.

+ References

No definition available.

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 21

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 55

-Paragraph 13

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- Definition

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.

+ References

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-Publisher FASB

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