| In millions, except per share items | Mar-31-22 | Dec-31-21 | Mar-31-21 | Dec-31-20 | Mar-31-20 | Dec-31-19 | Mar-31-19 | Dec-31-18 |
| | 8-K | 10-K | 8-K | 10-K | 8-K | 10-K | 8-K | 10-K |
| Operating revenues [Abstract] | | | | | | | | |
| Operating revenues | 856.9 | 828.2 | 301.3 | 744.4 | 278.5 | 746.4 | 701.8 | 706.1 |
| Operating revenues growth | 184.4% | 11.3% | 8.2% | -0.3% | -60.3% | 5.7% | | -6.5% |
| Operating expenses: | | | | | | | | |
| Cost of gas | | 292.3 | | 262.8 | | 254.9 | | 255.5 |
| Operations and maintenance | 209.5 | 204.2 | 52.2 | 180.1 | 48.9 | 178.2 | 168.7 | 156.7 |
| |
| Operations and maintenance | | | | | | | | |
| Environmental remediation | | 9.9 | | 9.7 | | 12.3 | | 11.1 |
| General taxes | | 38.6 | | 35.1 | | 32.4 | | 32.2 |
| Revenue taxes | 35.4 | | 12.7 | | | | | |
| Depreciation | 113.9 | 113.5 | 28.1 | 103.7 | | 91.5 | | 85.2 |
| Other operating expenses | | 3.9 | | 3.7 | | 3.3 | | 3.2 |
| Total operating expenses | 117.8 | 697.3 | 29.0 | 625.3 | 0.9 | 602.9 | 3.3 | 574.0 |
| Income from operations | 156.0 | 163.1 | 94.7 | 148.4 | 76.4 | 143.5 | 140.5 | 132.2 |
| Operating margin | 18.2% | 19.7% | 31.4% | 19.9% | 27.5% | 19.2% | 20.0% | 18.7% |
| Other income (expense), net | -10.0 | -12.6 | -3.5 | -13.9 | -3.6 | -22.8 | -16.5 | 7.5 |
| Interest expense, net | 44.9 | 44.5 | 11.1 | 43.1 | 10.5 | 42.7 | 38.0 | 37.1 |
| Income tax expense | 25.8 | 27.4 | 20.5 | 21.1 | 14.1 | 12.6 | 17.2 | 24.2 |
| Net income | 75.4 | 78.7 | 59.5 | 76.8 | 47.5 | 61.7 | 66.2 | 64.6 |
| |
| Other comprehensive income: | | | | | | | | |
| Amortization of non-qualified employee benefit plan liability, net of taxes | | 0.9 | | 0.7 | | 0.5 | | 0.8 |
| Comprehensive income | | 80.2 | | 74.6 | | 59.6 | | 65.8 |
| |
| Supplemental Income Statement Elements [Abstract] | | | | | | | | |
| |
| Average common shares outstanding: | | | | | | | | |
| Basic (in shares) | 30.9 | 30.7 | 30.7 | 30.5 | 30.4 | 29.8 | 28.9 | 28.8 |
| Diluted (in shares) | 30.9 | 30.8 | 30.7 | 30.6 | 30.4 | 29.9 | 28.9 | 28.9 |
| |
| Earnings per share of common stock: | | | | | | | | |
| Basic (in dollars per share) | | $2.56 | | $2.51 | | $2.07 | | $2.24 |
| Diluted (in dollars per share) | $2.44 | $2.56 | $1.94 | $2.51 | $1.56 | $2.07 | $2.29 | $2.24 |