Business Segments (Segment Financial Information) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Dec. 31, 2024
Segment Reporting Information [Line Items]
Revenues	$ 254.3	$ 226.4
Depreciation and amortization	29.2	22.6
Interest expense	23.9	20.3
Total assets	3,489.4		$ 3,417.4
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	3.7	4.2
Net purchased power	95.1	89.7
Operation and maintenance	65.5	60.9
Taxes other than income taxes	36.3	29.2
Other income / (expense), net	(0.6)	1.8
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	3.7	4.2
Income tax expense	1.0	1.6
Net Income (Loss) Attributable to Parent	2.7	2.6
Segment Reporting, Other Segment Item, Amount	0.6	(0.5)
Segment, Expenditure, Addition to Long-Lived Assets	85.3	174.6
Loss on asset disposal	0.0	1.3
Revenue from external customers
Segment Reporting Information [Line Items]
Revenues	254.3	226.4
Intersegment revenue
Segment Reporting Information [Line Items]
Revenues	0.0	0.0
Utility [Member]
Segment Reporting Information [Line Items]
Revenues	252.1	224.3
Operating Segments [Member] | Utility [Member]
Segment Reporting Information [Line Items]
Revenues	252.1	224.3
Depreciation and amortization	28.8	22.1
Interest expense	(14.7)	(11.2)
Net purchased power	94.8	89.5
Operation and maintenance	63.9	60.7
Taxes other than income taxes	36.2	29.2
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	12.4	11.7
Income tax expense	1.7	2.3
Net Income (Loss) Attributable to Parent	10.7	9.4
Segment Reporting, Other Segment Item, Amount	1.3	(0.1)
Segment, Expenditure, Addition to Long-Lived Assets	85.3	173.1
Operating Segments [Member] | Utility [Member] | Revenue from external customers
Segment Reporting Information [Line Items]
Revenues	251.8	224.1
Operating Segments [Member] | Utility [Member] | Intersegment revenue
Segment Reporting Information [Line Items]
Revenues	0.3	0.2
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Revenues	3.7	3.5
Depreciation and amortization	0.4	0.5
Interest expense	9.2	9.0
Total assets	56.8		62.2
Net purchased power	0.6	0.4
Operation and maintenance	2.8	1.4
Taxes other than income taxes	0.1	0.0
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	(8.7)	(7.5)
Income tax expense	(0.7)	(0.7)
Net Income (Loss) Attributable to Parent	(8.0)	(6.8)
Segment Reporting, Other Segment Item, Amount	(0.7)	(0.3)
Segment, Expenditure, Addition to Long-Lived Assets	0.0	1.5
Corporate, Non-Segment [Member] | Revenue from external customers
Segment Reporting Information [Line Items]
Revenues	2.5	2.3
Corporate, Non-Segment [Member] | Intersegment revenue
Segment Reporting Information [Line Items]
Revenues	1.2	1.2
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues	(1.5)	(1.4)
Depreciation and amortization	0.0	0.0
Interest expense	0.0	0.1
Net purchased power	(0.3)	(0.2)
Operation and maintenance	(1.2)	(1.2)
Taxes other than income taxes	0.0	0.0
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	0.0	0.0
Income tax expense	0.0	0.0
Net Income (Loss) Attributable to Parent	0.0	0.0
Segment Reporting, Other Segment Item, Amount	0.0	(0.1)
Segment, Expenditure, Addition to Long-Lived Assets	0.0	0.0
Intersegment Eliminations | Revenue from external customers
Segment Reporting Information [Line Items]
Revenues	0.0	0.0
Intersegment Eliminations | Intersegment revenue
Segment Reporting Information [Line Items]
Revenues	(1.5)	(1.4)
Subsidiaries [Member]
Segment Reporting Information [Line Items]
Revenues	252.1	224.3
Depreciation and amortization	28.8	22.1
Interest expense	14.7	11.2
Total assets	3,432.6		$ 3,355.2
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	12.4	11.7
Net purchased power	94.8	89.5
Operation and maintenance	63.9	60.7
Taxes other than income taxes	36.2	29.2
Other income / (expense), net	(1.3)	1.2
Income tax expense	1.7	2.3
Net Income (Loss) Attributable to Parent	$ 10.7	$ 9.4