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Coal Trading - Narrative (Details)	3 Months Ended	6 Months Ended
	Jun. 30, 2017 USD ($) agency	Jun. 30, 2017 USD ($)	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	$ 16,000,000	$ 16,000,000	$ 57,400,000
Initial margin posted	8,600,000	8,600,000	16,200,000
Margin in excess of the exchange-required variation and initial margin	1,500,000	1,500,000	2,000,000
Potential collateralization that may be requested by counterparties related to material adverse event	1,000,000	1,000,000	2,000,000
Collateral required to be posted to counterparties			1,000,000
Collateral posted to counterparties	0	$ 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		59.00%
External Credit Rating, Non Investment Grade [Member] | Credit Concentration Risk [Member] | Coal Trading Positions [Member] | Coal Trading
Concentration Risk [Line Items]
Concentration Risk, Percentage		32.00%
Non Rated [Member] | Credit Concentration Risk [Member] | Coal Trading Positions [Member] | Coal Trading
Concentration Risk [Line Items]
Concentration Risk, Percentage		9.00%
Cash Flow Hedging | Coal Trading
Concentration Risk [Line Items]
Total asset (liabilities) from coal trading activities, net	$ 0	$ 0	$ 0

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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.

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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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