| 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: | 59.6 | 54.6 | 53.3 | 51.1 | 47.9 | 48.6 | 41.0 | 40.7 |
| OPERATING REVENUES: growth | 9.2% | 2.4% | 4.4% | 6.5% | -1.4% | 18.6% | 0.7% | 1.6% |
| OPERATING EXPENSES: | | | | | | | | |
| Operation and maintenance | 14.1 | 11.8 | 10.8 | 10.4 | 9.7 | 8.9 | 8.0 | 8.1 |
| Administrative and general | 9.9 | 9.8 | 9.3 | 8.5 | 8.1 | 8.1 | 9.1 | 9.1 |
| Depreciation and amortization | 10.1 | 8.9 | 8.2 | 7.7 | 7.0 | 6.8 | 6.4 | 6.2 |
| Taxes other than income taxes | 1.4 | 1.3 | 1.2 | 1.2 | 1.1 | 1.1 | 1.1 | 1.1 |
| Operating expenses | 35.6 | 31.7 | 29.4 | 27.8 | 25.9 | 24.9 | 24.7 | 24.4 |
| Operating income | 24.5 | 23.4 | 24.4 | 23.8 | 22.5 | 23.7 | 22.9 | 22.7 |
| Operating margin | 41.1% | 42.9% | 45.8% | 46.6% | 47.0% | 48.8% | 55.9% | 55.7% |
| |
| OTHER INCOME (EXPENSES): | | | | | | | | |
| Interest on debt | -5.1 | -4.9 | -4.7 | -5.1 | -5.5 | -5.3 | -5.3 | -5.2 |
| Allowance for funds used during construction | 1.5 | 1.2 | 0.5 | 0.4 | 0.2 | 0.9 | 0.2 | 0.2 |
| Other pension costs | -1.3 | -1.2 | -1.4 | -1.5 | -1.3 | -1.2 | | |
| Other income (expenses), net | | -0.4 | -0.8 | -0.9 | -0.1 | -0.5 | -0.6 | -0.5 |
| Other income (expenses) | -4.9 | -5.3 | -5.8 | -7.1 | -6.7 | -6.2 | -5.6 | -5.4 |
| Income before income taxes | 19.6 | 18.1 | 18.6 | | | | | |
| Income tax (benefit) expense | 0.0 | 1.1 | 2.0 | 2.2 | 2.5 | 4.5 | 5.4 | 4.7 |
| Net Income | 19.6 | 17.0 | 16.6 | 14.4 | 13.4 | 13.0 | 11.8 | 12.5 |
| |
| Basic Earnings Per Share (in dollars per share) | $1.40 | $1.30 | $1.27 | $1.11 | $1.04 | $1.01 | $0.92 | $0.97 |
| Diluted Earnings Per Share (in dollars per share) | $1.40 | $1.30 | $1.27 | $1.11 | $1.04 | $1.01 | $0.92 | $0.97 |