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Summary of Significant Accounting Policies (Narrative) (Details) $ in Thousands, T in Millions | 12 Months Ended |
--- | --- | ---
Dec. 31, 2015 T | Dec. 31, 2014 | Dec. 31, 2013 USD ($) | Sep. 30, 2014
Revenue, Major Customer [Line Items] |  |  |  |
Error correction amount | $ |  |  | $ 483 |
Sales Revenue, Goods, Net [Member] |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |
Percentage of total revenue from four largest customers | 82.00% | 71.00% | 94.00% |
Coal Supply Commitment [Member] |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |
Coal supply commitment | 32 |  |  |
Year that supply commitments end | 2024 |  |  |
Priced coal supply commitment | 14 |  |  |
Savoy [Member] |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |
Equity method investment ownership percentage | 40.00% |  |  | 45.00%
Sunrise Energy [Member] |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |
Equity method investment ownership percentage | 50.00% |  |  |
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 |  |  |

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