| In millions, except per share items | Dec-31-19 | Sep-30-19 | Jun-30-19 | Mar-31-19 | Dec-31-18 | Sep-30-18 | Jun-30-18 | Mar-31-18 |
| | 10-K | 10-Q | 10-Q | 10-Q | 10-K | 10-Q | 10-Q | 10-Q |
| ASSETS | | | | | | | | |
| Current assets: | | | | | | | | |
| Cash and cash equivalents | 34.3 | 39.9 | 40.8 | 71.2 | 72.8 | 48.9 | 45.7 | 60.7 |
| Restricted cash | | 1.2 | 1.2 | 1.7 | 1.7 | 5.9 | 5.4 | 5.5 |
| Trade accounts and other receivables, net | 13.6 | 21.9 | 23.8 | 29.5 | 35.7 | 37.7 | 45.1 | 35.5 |
| |
| Inventories: | | | | | | | | |
| Finished goods | 31.4 | 36.2 | 39.7 | 35.5 | 41.4 | 48.2 | 55.5 | 53.6 |
| Raw materials and supplies | 16.3 | 21.1 | 18.7 | 19.2 | 22.6 | 25.7 | 22.0 | 20.5 |
| Total inventories | 47.7 | 57.3 | 58.4 | 54.7 | 64.0 | 73.9 | 77.5 | 74.0 |
| Prepaid expenses and other current assets | 3.4 | 5.0 | 4.9 | 4.4 | 4.8 | 5.4 | 5.9 | 5.4 |
| Income tax receivable | 3.1 | 3.1 | 2.3 | 2.4 | 2.3 | | | |
| Total current assets | 102.1 | 128.4 | 131.5 | 163.9 | 181.3 | 189.6 | 179.5 | 181.2 |
| Restricted cash | 9.7 | 9.2 | 8.9 | 8.9 | 8.8 | 3.8 | 3.9 | 3.9 |
| |
| Property, plant and equipment: | | | | | | | | |
| Land and land improvements | 11.5 | 36.8 | 39.6 | 39.6 | 39.6 | 39.6 | 41.6 | 41.6 |
| Land-use and mineral rights | 18.6 | 19.5 | 20.3 | 19.7 | 19.7 | 19.7 | 19.7 | 19.7 |
| Buildings | 23.0 | 75.7 | 76.0 | 75.8 | 75.8 | 75.8 | 72.4 | 72.4 |
| Machinery and equipment | 45.5 | 416.4 | 425.4 | 433.3 | 432.9 | 432.8 | 454.8 | 454.5 |
| Construction in progress | 0.0 | 27.1 | 28.8 | 28.9 | 29.1 | 29.6 | 36.3 | 36.1 |
| Total property, plant and equipment | 98.6 | 575.6 | 590.0 | 597.3 | 597.1 | 597.5 | 624.9 | 624.4 |
| Less accumulated depreciation and amortization | 35.2 | 335.0 | 333.2 | 330.8 | 323.5 | 316.2 | 317.2 | 308.7 |
| Net property, plant and equipment | 63.4 | 240.6 | 256.7 | 266.5 | 273.6 | 281.3 | 307.7 | 315.7 |
| Goodwill | | 3.5 | 3.5 | 3.5 | 3.5 | 3.5 | 3.5 | 3.5 |
| Operating lease right-of-use assets | 9.2 | 45.2 | 52.3 | 54.5 | | | | |
| Intangible and other assets, net | 7.1 | 11.8 | 11.4 | 7.7 | 7.2 | 6.3 | 7.1 | 10.6 |
| Total assets | 191.5 | 438.6 | 464.2 | 504.9 | 474.4 | 486.7 | 504.0 | 517.2 |
| |
| LIABILITIES AND SHAREHOLDERS’ EQUITY | | | | | | | | |
| Current liabilities: | | | | | | | | |
| Accounts payable | 7.6 | 11.2 | 13.9 | 12.6 | 12.2 | 15.4 | 18.1 | 20.5 |
| Accrued payroll and benefits | 2.8 | 4.7 | 4.0 | 3.8 | 7.0 | 6.4 | 5.4 | 3.8 |
| Accrued freight | 0.6 | 2.7 | 2.4 | 2.2 | 2.4 | 1.4 | 1.4 | 1.9 |
| Accrued utilities | 0.5 | 0.8 | 0.6 | 0.7 | 1.0 | 0.9 | 1.5 | 0.9 |
| Other accrued expenses | 18.6 | 15.2 | 7.8 | 7.5 | 14.0 | | | 10.9 |
| Notes payable, related parties | | | | 27.0 | 27.0 | 27.0 | 27.0 | |
| Long-term debt, current portion, net | 62.5 | | | 15.7 | 15.7 | | | |
| Operating lease liabilities | 54.7 | 11.5 | 11.5 | 10.6 | | | | |
| Other current liabilities | 5.0 | 2.2 | 0.7 | 0.7 | 1.2 | 10.1 | 9.1 | |
| Total current liabilities | 152.2 | 48.3 | 41.0 | 80.9 | 80.6 | 61.8 | 63.7 | 39.9 |
| Deferred income taxes | | 1.2 | 1.1 | 1.1 | 1.1 | 0.1 | 0.2 | 0.2 |
| Long-term debt, net | | 62.2 | 62.0 | 45.8 | 45.7 | 61.2 | 61.0 | 60.9 |
| |
| Noncurrent operating lease liabilities | | | | | | | | |
| Other long-term liabilities | | 3.7 | 4.2 | 4.2 | 10.8 | 6.5 | 5.9 | 5.1 |
| |
| Shareholders’ equity: | | | | | | | | |
| Preferred stock, par value $0.01 per share, 5,000 shares authorized, none outstanding | | | | | | | | |
| Common stock | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 | 0.3 |
| Additional paid-in capital | 139.6 | 138.9 | 138.3 | 136.2 | 132.1 | 131.4 | 130.6 | 126.6 |
| Retained earnings (accumulated deficit) | -100.6 | 138.0 | 167.4 | 183.7 | 203.9 | 225.5 | 242.3 | 257.1 |
| Total shareholders’ equity | 39.3 | 277.3 | 306.0 | 320.1 | 336.3 | 357.2 | 373.2 | 384.0 |
| Total liabilities and shareholders’ equity | 191.5 | 438.6 | 464.2 | 504.9 | 474.4 | 486.7 | 504.0 | 517.2 |