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Freelandville and Log Creek Purchases (Narrative) (Details) $ in Thousands |  | 12 Months Ended |
--- | --- | --- | ---
Dec. 12, 2016 USD ($) | Dec. 31, 2016 T | Dec. 31, 2014 USD ($) | Mar. 22, 2014 USD ($)
Business Acquisition [Line Items] |  |  |  |
Business acquisition purchase price | $ |  |  | $ 311,453 |
Freelandville Purchase [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Coal reserves acquired, tons |  | 14,200,000 |  |
Purchase price | $ |  |  |  | $ 18,250
Freelandville Purchase [Member] | Coal Supply Agreements [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Coal reserves acquired, tons |  | 1,435,000 |  |
Freelandville Purchase [Member] | Maximum [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Coal sales agreement, monthly amount of allowable adjustment up or down |  | 6,700 |  |
Log Creek [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Business acquisition purchase price | $ | $ 4,100 |  |  |
Log Creek [Member] | Coal Supply Agreements [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Coal reserves acquired, tons |  | 557,000 |  |
Sunrise Energy [Member] | Freelandville Purchase [Member] |  |  |  |
Business Acquisition [Line Items] |  |  |  |
Coal reserves acquired, tons |  | 1,600,000 |  |

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