| 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 |
| Operating Revenues | | | | | | | | |
| Regulated Energy | 429.4 | 383.9 | 352.7 | 343.0 | 345.3 | 326.3 | 305.7 | 301.9 |
| Total Operating Revenues | 680.7 | 570.0 | 488.2 | 479.6 | 490.3 | 449.6 | 498.9 | 459.2 |
| Total Operating Revenues growth | 19.4% | 16.7% | 1.8% | -2.2% | 9.0% | -9.9% | 8.6% | -7.9% |
| Operating Expenses | | | | | | | | |
| Propane and natural gas costs | 133.3 | 86.2 | 45.9 | | | | | |
| Operations | 164.5 | 148.3 | 142.1 | 137.8 | 132.5 | 121.9 | 115.7 | 107.6 |
| |
| Settlement Gain | | | | | | | | |
| Depreciation and amortization | 69.0 | 62.7 | 58.1 | 45.4 | 40.2 | 36.4 | 32.2 | 30.0 |
| Other taxes | 25.6 | 24.2 | 21.9 | 20.0 | 18.3 | 16.8 | 14.7 | 13.6 |
| Total Operating Expenses | 537.8 | 438.9 | 375.5 | 373.3 | 395.5 | 359.9 | 412.9 | 381.5 |
| Total operating income | 142.9 | 131.1 | 112.7 | 106.3 | 94.8 | 89.7 | 86.0 | 77.8 |
| Operating margin | 21.0% | 23.0% | 23.1% | 22.2% | 19.3% | 20.0% | 17.2% | 16.9% |
| Other income, net | 5.1 | 1.7 | 3.2 | -1.8 | -0.6 | -2.2 | -2.3 | 0.3 |
| Interest charges | -24.4 | -20.1 | -21.8 | -22.2 | -16.1 | -12.5 | -10.6 | -10.0 |
| Income Before Income Taxes | 123.6 | 112.7 | 94.2 | 82.2 | 78.1 | 75.0 | 73.0 | 68.0 |
| Income Taxes | 33.8 | 29.2 | 23.5 | 21.1 | 21.1 | 14.7 | 28.3 | 26.9 |
| Tax rate | 27.4% | 25.9% | 25.0% | 25.7% | 27.0% | 19.6% | 38.8% | 39.5% |
| |
| Discontinued Operation, Income (Loss) from Discontinued Operation During Phase-out Period, Net of Tax | | | | | | | | |
| Net Income | 89.8 | 83.5 | 71.5 | 65.2 | 56.6 | 58.1 | 44.7 | 41.1 |
| |
| Weighted Average Common Shares Outstanding: | | | | | | | | |
| Basic (shares) | 17.7 | 17.6 | 16.7 | 16.4 | 16.4 | 16.3 | 15.6 | 15.1 |
| Diluted (shares) | 17.8 | 17.6 | 16.8 | 16.4 | 16.4 | 16.4 | 15.6 | 15.1 |
| |
| Earnings Per Share of Common Stock: | | | | | | | | |
| Income (Loss) from Continuing Operations, Per Basic Share | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | | |
| |
| Discontinued Operation, Income (Loss) from Discontinued Operation, Net of Tax, Per Basic Share | | | | | | | | |
| Basic (in dollars per share) | $5.07 | $4.75 | $4.28 | $3.97 | $3.46 | $3.56 | $2.87 | $2.73 |
| |
| Earnings Per Share, Diluted [Abstract] | | | | | | | | |
| Income (Loss) from Continuing Operations, Per Diluted Share | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | 0.0 | | |
| |
| Discontinued Operation, Income (Loss) from Discontinued Operation, Net of Tax, Per Diluted Share | | | | | | | | |
| Diluted (in dollars per share) | $5.04 | $4.73 | $4.26 | $3.96 | $3.45 | $3.55 | $2.86 | $2.72 |