| In millions, except per share items | Jun-30-23 | Mar-31-23 | Sep-30-22 | Jun-30-22 | Mar-31-22 | Dec-31-21 | Sep-30-21 | Jun-30-21 |
| | 10-Q | 10-Q | 10-Q | 10-Q | 10-Q | 10-K | 10-Q | 10-Q |
| Operating revenues [Abstract] | | | | | | | | |
| Operating revenues | 230.0 | 466.8 | 110.0 | 201.0 | 346.8 | 288.9 | 94.8 | 133.4 |
| Operating revenues growth | 14.4% | 34.6% | 16.0% | 50.6% | 11.5% | 15.5% | 7.3% | 6.8% |
| Operating expenses: | | | | | | | | |
| Cost of gas | 102.4 | 205.7 | 36.1 | 79.7 | 145.6 | 113.6 | 25.3 | 41.2 |
| Operations and maintenance | 66.8 | 71.8 | 50.7 | 53.2 | 57.5 | 54.7 | 47.3 | 50.0 |
| |
| Operations and maintenance | | | | | | | | |
| Environmental remediation | 2.1 | 5.4 | 1.0 | 2.3 | 4.7 | 3.8 | 0.8 | 1.5 |
| General taxes | 10.9 | 14.2 | 9.6 | 9.0 | 12.1 | 9.3 | 9.1 | 8.9 |
| Revenue taxes | 9.2 | 19.0 | 4.4 | 8.2 | 13.4 | | 3.9 | 5.7 |
| Depreciation | 31.3 | 31.5 | 29.0 | 28.1 | 28.4 | 28.9 | 28.4 | 28.1 |
| Other operating expenses | 1.3 | 1.2 | 0.9 | 0.9 | 1.0 | 1.1 | 1.0 | 0.8 |
| Total operating expenses | 224.0 | 348.9 | 131.8 | 181.4 | 262.7 | 223.9 | 115.8 | 136.3 |
| Income from operations | 13.8 | 113.5 | -14.9 | 13.5 | 87.6 | 70.2 | -14.4 | 12.6 |
| Operating margin | 6.0% | 24.3% | -13.6% | 6.7% | 25.3% | 24.3% | -15.2% | 9.5% |
| Other income (expense), net | 6.6 | 7.0 | 1.6 | 2.5 | 3.7 | -4.2 | -2.2 | -2.6 |
| Interest expense, net | 19.0 | 18.3 | 13.1 | 11.6 | 11.5 | 11.2 | 11.2 | 11.0 |
| Income tax expense | 0.2 | 25.1 | -6.8 | 0.5 | 18.9 | 14.3 | -7.1 | -0.3 |
| Net income | 1.2 | 71.7 | -19.6 | 1.7 | 56.2 | 40.5 | -20.7 | -0.7 |
| |
| Other comprehensive income: | | | | | | | | |
| Amortization of non-qualified employee benefit plan liability, net of taxes | 0.1 | 0.1 | 0.2 | 0.2 | 0.2 | 0.2 | 0.2 | 0.2 |
| Comprehensive income | 2.6 | 71.4 | -19.4 | 1.9 | 56.4 | 41.4 | -20.4 | -0.5 |
| |
| Supplemental Income Statement Elements [Abstract] | | | | | | | | |
| Amortization of non-qualified employee benefit plan liability, tax | 0.0 | 0.0 | 0.1 | 0.1 | 0.1 | | 0.1 | 0.1 |
| |
| Average common shares outstanding: | | | | | | | | |
| Basic (in shares) | 36.0 | 35.6 | 34.9 | 34.3 | 31.2 | 30.7 | 30.7 | 30.7 |
| Diluted (in shares) | 36.1 | 35.7 | 34.9 | 34.4 | 31.2 | 30.8 | 30.7 | 30.7 |
| |
| Earnings per share of common stock: | | | | | | | | |
| Basic (in dollars per share) | $0.03 | $2.01 | ($0.56) | $0.05 | $1.80 | $1.32 | ($0.67) | ($0.02) |
| Diluted (in dollars per share) | $0.03 | $2.01 | ($0.56) | $0.05 | $1.80 | $1.32 | ($0.67) | ($0.02) |