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v3.19.1

Summary of Significant Accounting Policies (Narrative) (Details) $ / shares in Units, T in Millions | 1 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Jan. 31, 2019 USD ($) $ / shares | Mar. 31, 2018 USD ($) | Feb. 28, 2018 USD ($) | Dec. 31, 2018 USD ($) T | Dec. 31, 2017 USD ($) | Jan. 01, 2019 USD ($)
Revenue, Major Customer [Line Items] |  |  |  |  |  |
Allowance for doubtful accounts |  |  |  | $ 0 | $ 0 |
Surety bonds |  |  |  | 26,000,000 |  |
DD&A |  |  |  | 44,167,000 | $ 38,495,000 |
Subsequent Event [Member] |  |  |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |  |  |
Right-of-use asset |  |  |  |  |  | $ 500,000
Operating lease liabilities |  |  |  |  |  | $ 500,000
Dividends, date declared | 2019-01 |  |  |  |  |
Dividends payable, amount per share | $ / shares | $ 0.04 |  |  |  |  |
Dividends payable, date of record | Jan. 31, 2019 |  |  |  |  |
Dividends payable, date to be paid | Feb. 15, 2019 |  |  |  |  |
Gain on sale of investment | $ 2,500,000 |  |  |  |  |
Accounting Standards Update 2014-09 [Member] |  |  |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |  |  |
Revenue |  |  |  | 500,000 |  |
Accounting Standards Update 2016-01 [Member] |  |  |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |  |  |
Cumulative-effect adjustment from accumulated other comprehensive income to retained earnings |  |  |  | $ 1,100,000 |  |
Accounting Standards Update 2018 02 [Member] |  |  |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |  |  |
Cumulative-effect adjustment from accumulated other comprehensive income to retained earnings |  |  | $ 192,000 |  |  |
Sales Revenue, Goods, Net [Member] | Five Customers [Member] |  |  |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |  |  |
Percentage of revenue |  |  |  |  | 92.00% |
Sales Revenue, Goods, Net [Member] | Four Customers [Member] |  |  |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |  |  |
Percentage of revenue |  |  |  | 82.00% |  |
Accounts Receivable [Member] | Five Customers [Member] |  |  |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |  |  |
Percentage of revenue |  |  |  | 77.00% |  |
Accounts Receivable [Member] | Four Customers [Member] |  |  |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |  |  |
Percentage of revenue |  |  |  |  | 83.00% |
Coal Supply Commitment [Member] |  |  |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |  |  |
Coal supply commitment | T |  |  |  | 30.5 |  |
Year that supply commitments end |  |  |  | 2024 |  |
Priced coal supply commitment | T |  |  |  | 20.7 |  |
Minimum [Member] |  |  |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |  |  |
Discount rate |  |  |  | 5.00% |  |
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% |  |  |  |
Proceeds from the sale of investment |  | $ 8,000,000 |  |  |  |
Proceeds from the sale of investment, net commissions paid |  |  |  | $ 7,500,000 |  |
Sunrise Energy [Member] |  |  |  |  |  |
Revenue, Major Customer [Line Items] |  |  |  |  |  |
Equity method investment ownership percentage |  |  |  | 50.00% |  |

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