| In millions, except per share items | Dec-31-22 | Dec-31-21 | Dec-31-20 | Dec-31-19 | Dec-31-18 | Dec-31-17 | Dec-31-16 | Dec-31-15 |
| | 10-K | 10-K | 10-K | 10-K | 10-K | 10-K | 10-K | 10-K |
| Current assets: | | | | | | | | |
| Cash and cash equivalents | 3.0 | 2.5 | 8.0 | 8.8 | 15.5 | 12.5 | 9.8 | 15.9 |
| Restricted cash | 3.4 | 3.3 | 4.0 | 4.5 | 4.6 | 3.8 | 2.8 | |
| Accounts receivable | 29.9 | 13.6 | 14.4 | 25.6 | 18.4 | 16.8 | 22.3 | 16.7 |
| Inventory | 49.8 | 7.7 | 24.7 | 28.3 | 20.5 | 12.8 | | |
| Parts and supplies | 28.3 | 10.0 | 8.9 | 11.8 | | | | |
| Prepaid expenses | 4.5 | 2.1 | 3.3 | 1.7 | 11.4 | 5.5 | 9.6 | 1.2 |
| Total current assets | 138.5 | 39.2 | 63.3 | 82.4 | 85.0 | 67.7 | 82.8 | 66.6 |
| |
| Property, plant and equipment: | | | | | | | | |
| Land and mineral rights | 115.6 | 115.8 | 115.9 | 114.7 | 130.9 | 129.7 | 126.3 | 116.2 |
| Buildings and equipment | 534.1 | 342.8 | 352.1 | 351.6 | | | | |
| Mine development | 140.1 | 112.6 | 93.6 | 84.2 | 141.0 | 136.8 | 126.0 | 131.0 |
| Total property, plant and equipment | 789.8 | 571.2 | 561.6 | 550.5 | 637.4 | 988.2 | 340.0 | 348.0 |
| Less - accumulated depreciation, depletion and amortization | -309.4 | -268.4 | -252.2 | -220.8 | -224.7 | -203.4 | -169.6 | -150.0 |
| Total property, plant and equipment, net | 480.5 | 302.8 | 309.4 | 329.7 | 412.6 | 424.0 | | |
| Investment in Sunrise Energy | 4.0 | 3.5 | 3.2 | 3.1 | 3.7 | | | |
| Other assets | 7.6 | 8.4 | 8.3 | 10.3 | 14.2 | 14.7 | 14.1 | 11.4 |
| Total Assets | 630.6 | 354.0 | 384.1 | 425.6 | 515.5 | 518.2 | 531.3 | 540.4 |
| |
| Current liabilities: | | | | | | | | |
| Current portion of bank debt, net | 33.0 | 23.1 | 34.3 | 33.0 | | | | |
| Accounts payable and accrued liabilities | 83.0 | 41.5 | 31.4 | 31.8 | 26.4 | 21.1 | 19.9 | 26.2 |
| |
| Deferred revenue | | | | | | | | |
| Contract liability - power purchase agreement and capacity payment reduction | | | | | | | | |
| Total current liabilities | 239.6 | 64.6 | 71.2 | 64.8 | 51.8 | 54.3 | 48.7 | 51.0 |
| |
| Long-term liabilities: | | | | | | | | |
| Long-term bank debt, excluding current maturities, net | 49.7 | 84.7 | 97.3 | 140.6 | | | | |
| Convertible note payable | 10.0 | | | | | | | |
| Convertible notes payable - related party | 9.0 | | | | | | | |
| Deferred income taxes | 4.6 | 2.9 | 2.8 | 4.9 | 26.4 | 28.7 | 45.2 | 49.0 |
| Asset retirement obligations | 38.1 | 42.3 | 48.7 | 47.2 | 43.9 | 13.5 | 13.1 | 12.2 |
| Other | 1.3 | 1.6 | 2.8 | 4.1 | 8.1 | 6.6 | 2.5 | 1.8 |
| Total long-term liabilities | 175.9 | 234.5 | 279.9 | 362.0 | 438.9 | 214.6 | 265.7 | 568.8 |
| Total liabilities | 415.5 | 167.7 | 194.9 | 230.1 | 256.6 | 268.9 | 314.4 | 333.6 |
| |
| Commitments and contingencies | | | | | | | | |
| Stockholders' equity: | | | | | | | | |
| Preferred stock, $.10 par value, 10,000 shares authorized; none issued and outstanding | | | | | | | | |
| Common stock | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 |
| Additional paid-in capital | 118.8 | 104.1 | 103.4 | 102.2 | 100.7 | 97.9 | 93.8 | 92.3 |
| Retained earnings | 95.9 | 77.8 | 81.6 | 89.0 | 153.8 | 150.2 | 122.1 | 114.3 |
| Total stockholders’ equity | 215.0 | 182.2 | 185.3 | 191.5 | 254.9 | 249.3 | 216.9 | 206.8 |
| Total liabilities and stockholders’ equity | 630.6 | 354.0 | 384.1 | 425.6 | 515.5 | 518.2 | 531.3 | 540.4 |