| 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 |
| SALES AND OPERATING REVENUES: | | | | | | | | |
| Revenues | | | | | | | | |
| Other revenues | 6.4 | 3.8 | 2.2 | 6.0 | | | | |
| Total revenue | 362.0 | 247.7 | 244.2 | 323.5 | 293.6 | 271.6 | 281.5 | 340.1 |
| Total revenue growth | 46.2% | 1.4% | -24.5% | 10.2% | 8.1% | -3.5% | -17.2% | 41.0% |
| EXPENSES: | | | | | | | | |
| Operating expenses | 266.6 | 198.8 | 186.0 | 247.9 | 215.4 | | | |
| Depreciation, depletion and amortization | 46.9 | 40.0 | 39.6 | 48.6 | 44.2 | 38.5 | 35.6 | 43.9 |
| Asset retirement obligations accretion | 1.0 | 1.5 | 1.4 | 1.3 | | | | |
| Exploration costs | 0.7 | 0.5 | 0.8 | 1.2 | 1.2 | | | |
| General and administrative | 21.3 | 17.7 | 13.5 | 15.9 | 14.8 | 17.5 | 12.8 | 17.2 |
| Total operating expenses | 697.4 | 253.7 | 482.8 | 405.7 | 290.0 | 257.8 | 273.0 | 312.6 |
| INCOME (LOSS) FROM OPERATIONS | 30.4 | -6.0 | 3.1 | | | | | |
| Operating margin | 8.4% | -2.4% | 1.3% | | | | | |
| Interest expense (1) | -11.0 | -8.0 | -13.0 | -16.0 | -16.3 | -12.4 | -14.8 | -15.6 |
| Equity method investment (loss) income | 0.4 | 0.4 | 1.1 | | | | -1.4 | -1.6 |
| NET INCOME (LOSS) BEFORE INCOME TAXES | 19.9 | -3.7 | -8.9 | -82.2 | 3.5 | 13.9 | 8.5 | 27.6 |
| |
| INCOME TAX EXPENSE: | | | | | | | | |
| Current | | | -0.6 | -0.5 | -1.8 | -2.7 | -0.2 | 0.0 |
| Deferred | 1.8 | 0.0 | -2.1 | -21.8 | -2.3 | -16.4 | -3.9 | 7.5 |
| Total income tax expense | 1.8 | 0.0 | -2.7 | -22.3 | -4.1 | -19.2 | -4.0 | 7.4 |
| NET INCOME (LOSS) | 18.1 | -3.8 | -6.2 | -59.9 | 7.6 | 33.1 | 12.5 | 20.1 |
| |
| NET INCOME (LOSS) PER SHARE: | | | | | | | | |
| Basic (in dollars per share) | $0.57 | ($0.12) | ($0.20) | ($1.95) | $0.25 | $1.08 | $0.42 | $0.68 |
| Diluted (in dollars per share) | $0.55 | ($0.12) | ($0.20) | ($1.95) | $0.25 | $1.08 | $0.42 | $0.68 |
| |
| WEIGHTED AVERAGE SHARES OUTSTANDING | | | | | | | | |
| Basic (in shares) | 32.0 | 30.6 | 30.4 | 30.3 | 30.1 | 29.7 | 29.3 | 29.0 |
| Diluted (in shares) | 33.6 | 30.6 | 30.4 | 30.3 | 30.1 | 29.7 | 29.3 | 29.0 |
| |
| (1) Interest Expense: | | | | | | | | |
| Interest on bank debt | 7.6 | 8.5 | 10.7 | 11.5 | 11.9 | | | |
| Other interest | 0.7 | | 0.0 | 0.2 | 0.2 | | | |
| |
| Amortization and swap-related interest: | | | | | | | | |
| Payments on interest rate swap, net of changes in value | -0.9 | -3.0 | | | | | | |
| Amortization of debt issuance costs | 3.6 | 2.6 | 2.3 | 2.1 | 2.0 | | | |
| Total amortization and swap related interest | 2.7 | -0.5 | | | | | | |
| Total interest expense | 11.0 | 8.0 | 13.0 | 16.0 | 16.3 | 12.4 | 14.8 | 15.6 |