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Coal Trading (Details) - USD ($) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Sep. 30, 2016
Coal trading derivative instruments and balance sheet offsetting disclosures: |  |  |  |  |
Net assets (liabilities) from coal trading activities |  |  |  |  | $ 0
Coal Trading [Member] |  |  |  |  |
Trading revenue |  |  |  |  |
Trading revenue |  | $ (10,900,000) | $ 42,800,000 | $ 58,400,000 |
Coal trading derivative instruments and balance sheet offsetting disclosures: |  |  |  |  |
Assets from coal trading activities, gross amounts of recognized assets |  | 191,200,000 | 128,600,000 |  |
Liabilities from coal trading activities, gross amounts of recognized liabilities |  | (249,100,000) | (110,000,000) |  |
Assets and (liabilities) from coal trading activities, net amounts recognized before the application of variation margin |  | (57,900,000) | 18,600,000 |  |
Gross amounts of coal trading liabilities offset against associated coal trading assets |  | (190,500,000) | (87,300,000) |  |
Gross amounts of coal trading assets offset against associated coal trading liabilities |  | 190,500,000 | 87,300,000 |  |
Net coal trading assets (liabilities) offset against associated (liabilities) assets |  | 0 | 0 |  |
Variation margin held offset against assets from coal trading activities |  | 0 | (17,800,000) |  |
Variation margin posted offset against liabilities from coal trading activities |  | 57,400,000 | 7,100,000 |  |
Net variation margin (held) posted | [1] | 57,400,000 | 10,700,000 |  |
Assets from coal trading activities, net |  | 700,000 | 23,500,000 |  |
Liabilities from coal trading activities, net |  | (1,200,000) | (15,600,000) |  |
Net assets (liabilities) from coal trading activities |  | (500,000) | 7,900,000 |  |
Commodity futures, swaps and options [Member] | Coal Trading [Member] |  |  |  |  |
Trading revenue |  |  |  |  |
Trading revenue |  | (96,500,000) | 107,300,000 | 92,300,000 |
Physical commodity purchase / sale contracts [Member] | Coal Trading [Member] |  |  |  |  |
Trading revenue |  |  |  |  |
Trading revenue |  | $ 85,600,000 | (64,500,000) | $ (33,900,000) |
Cash Flow Hedging [Member] | Coal Trading [Member] |  |  |  |  |
Coal trading derivative instruments and balance sheet offsetting disclosures: |  |  |  |  |
Net variation margin (held) posted |  |  | $ 0 |  |
[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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