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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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