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Coal Trading (Details 2) $ in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Concentration Risk [Line Items] |  |  |
Number of major credit rating agencies that adjusted corporate credit rating |  | 3 |
Coal Trading [Member] |  |  |
Concentration Risk [Line Items] |  |  |
Potential collateralization that may be requested by counterparties related to material adverse event |  | $ 2.0 | $ 21.0
Margin posted to counterparties related to material adverse event |  | 1.0 | 0.0
Additional potential collateral requirements for a credit downgrade |  | 0.0 | 0.0
Margin posted to counterparties related to credit rating |  | 0.0 | 0.0
Net variation margin held | [1] | (57.4) | (10.7)
Initial margin posted |  | 16.2 | 9.2
Margin in excess of the exchange-required variation and initial margin |  | $ 2.0 | $ 0.7
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 |  | 22.00% |
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 |  | 7.00% |
Non Rated [Member] | Credit Concentration Risk [Member] | Coal Trading Positions [Member] | Coal Trading [Member] |  |  |
Concentration Risk [Line Items] |  |  |
Credit concentration risk percentage |  | 71.00% |
[1] | None of the net variation margin (held) posted at December 31, 2016 and 2015, respectively, related to cash flow hedges.

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