| 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 |
| Net sales | 1,717.3 | 1,407.6 | 1,343.8 | 1,430.9 | 1,957.0 | 940.4 | 868.7 | 941.4 |
| Net sales growth | 22.0% | 4.7% | -6.1% | -26.9% | 108.1% | 8.3% | -7.7% | -1.7% |
| Cost of sales | -1,594.2 | -1,332.8 | -1,280.4 | -1,377.5 | -1,665.8 | -799.7 | -682.6 | -738.9 |
| Gross margin | 123.1 | 74.7 | 63.4 | 53.5 | 291.1 | 140.8 | 186.2 | 202.5 |
| Gross margin | 7.2% | 5.3% | 4.7% | 3.7% | 14.9% | 15.0% | 21.4% | 21.5% |
| Selling, general and administrative expense | 91.5 | 75.8 | 77.9 | 80.8 | 105.0 | 79.6 | 37.2 | 47.7 |
| Foreign exchange gain (loss) | 4.7 | 0.9 | -5.7 | -3.2 | | | | |
| Other operating expense, net | -10.2 | -10.3 | -10.2 | -22.0 | -12.4 | -1.3 | -5.7 | -35.3 |
| Operating income (loss) | 26.1 | -10.4 | -30.4 | -52.5 | 147.8 | 59.0 | 143.3 | 119.5 |
| Operating margin | 1.5% | -0.7% | -2.3% | -3.7% | 7.6% | 6.3% | 16.5% | 12.7% |
| Interest expense | -66.2 | -66.4 | -55.5 | -52.0 | -55.9 | -40.0 | -34.6 | -36.9 |
| Other components of pension and OPEB, excluding service costs (Note 14) | 5.0 | -4.3 | 2.6 | | | | | |
| Gain (loss) on GreenFirst equity securities (Note 2) | 5.2 | -3.6 | | | | | | |
| Other income, net | 5.4 | 7.5 | -7.1 | 17.8 | 13.4 | 2.4 | 0.7 | 0.2 |
| Loss from continuing operations before income taxes | -23.8 | -82.9 | -98.2 | -104.3 | 126.5 | 342.7 | 112.6 | 82.9 |
| Income tax (expense) benefit (Note 15) | -0.9 | 34.7 | 60.9 | 20.4 | -27.0 | -19.2 | -39.3 | -27.6 |
| Tax rate | | 41.9% | 62.0% | 19.5% | 21.4% | 5.6% | 34.9% | 33.3% |
| Equity in loss of equity method investment | -2.7 | -1.6 | -0.7 | | | | | |
| Loss from continuing operations | -27.4 | -49.8 | -38.1 | -83.9 | 99.5 | 323.4 | 73.3 | 55.3 |
| Income (loss) from discontinued operations, net of taxes (Note 2) | 12.5 | 116.2 | 38.6 | 61.5 | 29.0 | 1.5 | | |
| Net loss | -14.9 | 66.4 | 0.6 | -22.5 | 128.4 | 325.0 | 73.3 | 55.3 |
| |
| Basic and Diluted earnings per common share (Note 12) | | | | | | | | |
| Loss from continuing operations-basic (in dollars per share) | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | | |
| Income (loss) from discontinued operations-basic (in dollars per share) | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | | |
| |
| Net loss per common share-basic (in dollars per share) | ($0.23) | $1.05 | $0.01 | ($0.57) | $2.27 | $7.17 | $1.61 | $1.31 |
| Loss from continuing operations-diluted (in dollars per share) | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | | |
| Income (loss) from discontinued operations-diluted (in dollars per share) | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | | |
| Net loss per common share-diluted (in dollars per share) | ($0.23) | $1.05 | $0.01 | ($0.57) | $1.96 | $5.81 | $1.55 | $1.30 |