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Summary of Significant Accounting Policies (Narrative) (Details) $ in Thousands, T in Millions | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2016 USD ($) agreement T | Dec. 31, 2015 USD ($) | Dec. 31, 2014 USD ($) | Nov. 03, 2016 | Nov. 02, 2016 | Oct. 01, 2014 | Sep. 30, 2014
Revenue, Major Customer [Line Items] |  |  |  |  |  |  |
DD&A | $ 35,565 | $ 43,942 | $ 29,262 |  |  |  |
Interest swaps, net | (200) |  |  |  |  |  |
Fair value of long term asset | 300 |  |  |  |  |  |
Fair value of current liability | $ 500 |  |  |  |  |  |
Number of interest rate swaps | agreement | 2 |  |  |  |  |  |
Interest Rate Swap [Member] |  |  |  |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |  |  |  |
Interest swaps, net | $ (200) |  |  |  |  |  |
Interest Rate Swap One [Member] |  |  |  |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |  |  |  |
Notional value of interest rate swap | 125,000 |  |  |  |  |  |
Interest Rate Swap Two [Member] |  |  |  |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |  |  |  |
Notional value of interest rate swap | 60,000 |  |  |  |  |  |
Change from Straight Line Method to Units of Production Method [Member] |  |  |  |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |  |  |  |
Reduction to depreciation, depletion, and amortization | $ 2,600 |  |  |  |  |  |
Sales Revenue, Goods, Net [Member] | Five Customers [Member] |  |  |  |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |  |  |  |
Percentage of total revenue from largest customers | 90.00% |  |  |  |  |  |
Sales Revenue, Goods, Net [Member] | Four Customers [Member] |  |  |  |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |  |  |  |
Percentage of total revenue from largest customers |  | 82.00% | 71.00% |  |  |  |
Coal Supply Commitment [Member] |  |  |  |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |  |  |  |
Coal supply commitment | T | 29 |  |  |  |  |  |
Year that supply commitments end | 2024 |  |  |  |  |  |
Priced coal supply commitment | T | 13 |  |  |  |  |  |
Minimum [Member] |  |  |  |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |  |  |  |
Discount rate | 5.50% |  |  |  |  |  |
Estimated useful lives | P3Y |  |  |  |  |  |
Maximum [Member] |  |  |  |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |  |  |  |
Discount rate | 10.00% |  |  |  |  |  |
Estimated useful lives | P25Y |  |  |  |  |  |
Savoy [Member] |  |  |  |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |  |  |  |
Equity method investment ownership percentage | 30.60% |  |  | 30.60% | 40.80% | 40.80% | 45.00%
Sunrise Energy [Member] |  |  |  |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |  |  |  |
Equity method investment ownership percentage | 50.00% |  |  |  |  |  |
Carlisle Assets [Member] |  |  |  |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |  |  |  |
Reduction to depreciation, depletion, and amortization | $ 4,400 |  |  |  |  |  |

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