| 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 | 5.6 | 5.8 | 6.4 | 6.3 | 5.6 | 4.8 | 3.8 | 3.4 |
| Total revenue | 586.7 | 491.4 | 362.0 | 275.4 | 270.2 | 259.9 | 247.7 | 247.8 |
| Total revenue growth | 117.2% | 89.1% | 46.2% | 11.1% | 15.8% | 13.1% | 0.9% | -4.2% |
| EXPENSES: | | | | | | | | |
| Operating expenses | 409.6 | 345.5 | 266.6 | 225.1 | 228.4 | 219.4 | 198.8 | 199.0 |
| Depreciation, depletion and amortization | 61.3 | 55.3 | 46.9 | 42.0 | 40.6 | 39.2 | 40.0 | 39.3 |
| Asset retirement obligations accretion | 1.4 | 1.2 | 1.0 | 1.1 | 1.3 | 1.4 | 1.5 | 1.5 |
| Exploration costs | 0.9 | 0.8 | 0.7 | 0.6 | 0.5 | 0.5 | 0.5 | 0.4 |
| General and administrative | 27.0 | 25.1 | 21.3 | 18.9 | 18.4 | 18.1 | 17.7 | 14.0 |
| Total operating expenses | 1,184.7 | 951.7 | 697.4 | 282.9 | 284.4 | 273.7 | 253.7 | 494.0 |
| INCOME (LOSS) FROM OPERATIONS | 91.4 | 68.3 | 30.4 | -7.5 | -14.2 | -13.9 | -6.0 | -4.5 |
| Operating margin | 15.6% | 13.9% | 8.4% | -2.7% | -5.3% | -5.3% | -2.4% | -1.8% |
| Interest expense (1) | -14.3 | -13.1 | -11.0 | -9.3 | -8.1 | -7.9 | -8.0 | -8.3 |
| Equity method investment (loss) income | 0.0 | 0.4 | 0.4 | 0.7 | 0.6 | 0.5 | 0.4 | 0.0 |
| NET INCOME (LOSS) BEFORE INCOME TAXES | 77.0 | 55.6 | 19.9 | -16.1 | -11.7 | -11.3 | -3.7 | -2.8 |
| |
| INCOME TAX EXPENSE: | | | | | | | | |
| Current | 0.5 | 0.4 | | | | | | |
| Deferred | 5.9 | 4.9 | 1.8 | 3.6 | 1.6 | 1.6 | 0.0 | -2.1 |
| Total income tax expense | 6.4 | 5.3 | 1.8 | 3.6 | 1.6 | 1.6 | 0.0 | -2.1 |
| NET INCOME (LOSS) | 39.0 | 18.7 | | | -17.3 | -16.9 | -7.8 | |
| |
| NET INCOME (LOSS) PER SHARE: | | | | | | | | |
| Basic (in dollars per share) | $2.18 | $1.56 | $0.56 | ($0.64) | ($0.43) | ($0.42) | ($0.12) | ($0.02) |
| Diluted (in dollars per share) | $2.06 | $1.48 | $0.54 | ($0.64) | ($0.43) | ($0.42) | ($0.12) | ($0.02) |
| |
| WEIGHTED AVERAGE SHARES OUTSTANDING | | | | | | | | |
| Basic (in shares) | 32.8 | 32.2 | 31.7 | 31.3 | 30.7 | 30.7 | 30.6 | 30.6 |
| Diluted (in shares) | 35.1 | 33.6 | 32.1 | 31.4 | 30.7 | 30.7 | 30.6 | 30.6 |
| |
| (1) Interest Expense: | | | | | | | | |
| Interest on bank debt | 8.5 | 8.1 | 7.6 | 7.5 | 7.5 | 8.1 | 8.5 | 9.1 |
| Other interest | 1.6 | 1.1 | 0.7 | 0.3 | 0.1 | | | |
| |
| Amortization and swap-related interest: | | | | | | | | |
| Payments on interest rate swap, net of changes in value | | | | | -2.3 | -2.0 | | |
| Amortization of debt issuance costs | 4.3 | 4.1 | 3.6 | 3.2 | 2.8 | 2.6 | 2.6 | 2.5 |
| Total amortization and swap related interest | 4.3 | 3.9 | 2.7 | 1.6 | 0.5 | 0.0 | | |
| Total interest expense | 14.3 | 13.1 | 11.0 | 9.3 | 8.1 | 7.9 | 8.0 | 8.3 |