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

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- Definition

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

+ References

No definition available.

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- Definition

The aggregate fair value of assets needed to immediately settle the derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period.

+ References

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- Definition

The aggregate fair value of assets that are already posted, at the end of the reporting period, as collateral for derivative instruments with credit-risk-related contingent features.

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- Definition

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.

+ References

No definition available.

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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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- Definition

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

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The net amount as of the balance sheet date of the fair value of derivative assets and derivative liabilities that in accordance with the entity's accounting policy were offset against collateral under a master netting arrangement.

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Coal Trading - Narrative (Details)	3 Months Ended	9 Months Ended
Coal Trading - Narrative (Details)	Jun. 30, 2017 agency	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Concentration Risk [Line Items]
Number of major credit rating agencies that adjusted corporate credit rating \| agency	2
Coal Trading
Concentration Risk [Line Items]
Total asset (liabilities) from coal trading activities, net		$ 31,900,000	$ 57,400,000
Initial margin posted		14,600,000	16,200,000
Potential collateralization that may be requested by counterparties related to material adverse event		0	2,000,000
Collateral required to be posted to counterparties			1,000,000
Collateral posted to counterparties		$ 0	0
External Credit Rating, Investment Grade [Member] \| Credit Concentration Risk [Member] \| Coal Trading Positions [Member] \| Coal Trading
Concentration Risk [Line Items]
Concentration Risk, Percentage		33.00%
Non Rated [Member] \| Credit Concentration Risk [Member] \| Coal Trading Positions [Member] \| Coal Trading
Concentration Risk [Line Items]
Concentration Risk, Percentage		67.00%
Cash Flow Hedging \| Coal Trading
Concentration Risk [Line Items]
Total asset (liabilities) from coal trading activities, net		$ 0	$ 0