| In millions, except per share items | Jun-30-23 | Mar-31-23 | Dec-31-22 | Sep-30-22 | Jun-30-22 | Mar-31-22 | Dec-31-21 | Sep-30-21 |
| | 10-Q | 10-Q | 10-K | 10-Q | 10-Q | 10-Q | 10-K | 10-Q |
| SALES AND OPERATING REVENUES: | | | | | | | | |
| Revenues | | | | | | | | |
| Other revenues | 1.6 | 1.3 | 1.2 | 1.5 | 1.8 | 1.9 | 1.1 | 0.8 |
| Total revenue | 161.2 | 188.3 | 152.1 | 85.1 | 65.9 | 58.9 | 65.5 | 79.8 |
| Total revenue growth | 144.5% | 219.7% | 132.1% | 6.6% | 18.5% | 26.2% | -0.2% | 22.3% |
| EXPENSES: | | | | | | | | |
| Operating expenses | 115.4 | 133.5 | 96.1 | 64.6 | 51.4 | 54.6 | 54.6 | 67.8 |
| Depreciation, depletion and amortization | 17.2 | 18.0 | 15.0 | 11.2 | 11.2 | 9.5 | 10.1 | 9.8 |
| Asset retirement obligations accretion | 0.5 | 0.5 | 0.3 | 0.3 | 0.3 | 0.2 | 0.4 | 0.4 |
| Exploration costs | 0.3 | 0.2 | 0.3 | 0.1 | 0.2 | 0.1 | 0.2 | 0.1 |
| General and administrative | 5.6 | 6.9 | 10.9 | 3.6 | 3.7 | 3.1 | 8.5 | 3.1 |
| Total operating expenses | 299.7 | 321.9 | 483.4 | 79.7 | 66.7 | 67.6 | 68.9 | 81.2 |
| INCOME (LOSS) FROM OPERATIONS | 22.2 | 29.2 | 34.5 | 5.4 | -0.8 | -8.7 | -3.4 | -1.4 |
| Operating margin | 13.8% | 15.5% | 22.7% | 6.3% | -1.2% | -14.7% | -5.2% | -1.7% |
| Interest expense (1) | -3.5 | -3.9 | -3.5 | -3.4 | -2.3 | -1.8 | -1.9 | -2.1 |
| Equity method investment (loss) income | -0.2 | 0.1 | -0.1 | 0.2 | 0.2 | 0.2 | 0.2 | 0.1 |
| NET INCOME (LOSS) BEFORE INCOME TAXES | 18.5 | 25.4 | 30.9 | 2.2 | -3.0 | -10.3 | -5.0 | 6.6 |
| |
| INCOME TAX EXPENSE: | | | | | | | | |
| Current | 0.1 | 0.4 | | | | | | |
| Deferred | 1.5 | 2.9 | 0.9 | 0.6 | 0.4 | -0.2 | 2.7 | -1.4 |
| Total income tax expense | 1.6 | 3.4 | 0.9 | 0.6 | 0.4 | -0.2 | 2.7 | -1.4 |
| NET INCOME (LOSS) | 16.9 | 22.1 | | | -3.4 | -10.1 | -3.8 | |
| |
| NET INCOME (LOSS) PER SHARE: | | | | | | | | |
| Basic (in dollars per share) | $0.51 | $0.67 | $0.95 | $0.05 | ($0.11) | ($0.33) | ($0.25) | $0.26 |
| Diluted (in dollars per share) | $0.47 | $0.61 | $0.93 | $0.05 | ($0.11) | ($0.33) | ($0.25) | $0.26 |
| |
| WEIGHTED AVERAGE SHARES OUTSTANDING | | | | | | | | |
| Basic (in shares) | 33.1 | 33.0 | 32.0 | 33.0 | 30.8 | 30.8 | 30.6 | 30.6 |
| Diluted (in shares) | 36.7 | 36.7 | 33.6 | 33.3 | 30.8 | 30.8 | 30.6 | 30.6 |
| |
| (1) Interest Expense: | | | | | | | | |
| Interest on bank debt | 2.1 | 2.3 | 2.0 | 2.1 | 1.7 | 1.7 | 1.9 | 2.2 |
| Other interest | 0.5 | 0.4 | 0.4 | 0.2 | 0.1 | | | |
| |
| Amortization and swap-related interest: | | | | | | | | |
| Payments on interest rate swap, net of changes in value | | | | | -0.3 | -0.6 | -0.7 | -0.7 |
| Amortization of debt issuance costs | 1.0 | 1.2 | 1.1 | 1.0 | 0.8 | 0.7 | 0.7 | 0.7 |
| Total amortization and swap related interest | 1.0 | 1.2 | 1.1 | 1.0 | 0.6 | 0.1 | 0.0 | -0.1 |
| Total interest expense | 3.5 | 3.9 | 3.5 | 3.4 | 2.3 | 1.8 | 1.9 | 2.1 |