Business Segments (Segment Financial Information) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2025	Mar. 31, 2025	Jun. 30, 2024	Mar. 31, 2024	Jun. 30, 2025	Jun. 30, 2024	Dec. 31, 2024
Segment Reporting Information [Line Items]
Revenues	$ 229.9		$ 209.2		$ 484.2	$ 435.6
Depreciation and amortization	29.6		21.1		58.8	43.7
Interest expense	19.9		20.8		43.8	41.1
Total assets	3,521.0				3,521.0		$ 3,417.4
Income (Loss) from Continuing Operations before Income Taxes, Noncontrolling Interest	5.3		11.5		9.0	15.7
Net purchased power	82.8		69.5		177.9	159.2
Operation and maintenance	65.5		61.1		131.0	122.0
Taxes other than income taxes	27.8		27.1		64.1	56.3
Other income, net	1.0		1.9		0.4	3.7
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	5.3		11.5		9.0	15.7
Income tax expense	3.0		1.3		4.0	2.9
Net Income (Loss) Attributable to Parent	0.7	$ 2.7	10.2	$ 2.6	3.4	12.8
Segment Reporting, Other Segment Item, Amount	(1.0)		(1.9)		(0.4)	(2.4)
Segment, Expenditure, Addition to Long-Lived Assets	73.0		114.5		158.3	289.1
Net income	2.3		10.2		5.0	12.8
Revenue from external customers
Segment Reporting Information [Line Items]
Revenues	229.9		209.2		484.2	435.6
Intersegment revenue
Segment Reporting Information [Line Items]
Revenues	0.0		0.0		0.0	0.0
Utility [Member]
Segment Reporting Information [Line Items]
Revenues	227.6		206.9		479.7	431.2
Operating Segments [Member] | Utility [Member]
Segment Reporting Information [Line Items]
Revenues	227.6		206.9		479.7	431.2
Depreciation and amortization	29.0		20.7		57.8	42.8
Interest expense	(13.7)		(11.7)		(28.4)	(22.9)
Net purchased power	82.5		69.2		177.3	158.7
Operation and maintenance	64.8		60.8		128.7	121.5
Taxes other than income taxes	27.7		26.9		63.9	56.1
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	9.5		18.9		21.9	30.6
Income tax expense	4.0		3.8		5.7	6.1
Net Income (Loss) Attributable to Parent			15.1			24.5
Segment Reporting, Other Segment Item, Amount	0.4		(1.3)		1.7	(1.4)
Segment, Expenditure, Addition to Long-Lived Assets	72.2		115.0		157.5	288.1
Net income	5.5				16.2
Operating Segments [Member] | Utility [Member] | Revenue from external customers
Segment Reporting Information [Line Items]
Revenues	227.4		206.7		479.2	430.8
Operating Segments [Member] | Utility [Member] | Intersegment revenue
Segment Reporting Information [Line Items]
Revenues	0.2		0.2		0.5	0.4
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Revenues	3.7		3.8		7.4	7.3
Depreciation and amortization	0.6		0.4		1.0	0.9
Interest expense	6.2		9.0		15.4	18.0
Total assets	58.4				58.4		62.2
Net purchased power	0.5		0.6		1.1	1.0
Operation and maintenance	1.9		1.5		4.7	2.9
Taxes other than income taxes	0.1		0.2		0.2	0.2
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	(4.2)		(7.4)		(12.9)	(14.9)
Income tax expense	(1.0)		(2.5)		(1.7)	(3.2)
Net Income (Loss) Attributable to Parent			(4.9)			(11.7)
Segment Reporting, Other Segment Item, Amount	(1.4)		(0.5)		(2.1)	(0.8)
Segment, Expenditure, Addition to Long-Lived Assets	0.8		(0.5)		0.8	1.0
Net income	(3.2)				(11.2)
Corporate, Non-Segment [Member] | Revenue from external customers
Segment Reporting Information [Line Items]
Revenues	2.5		2.5		5.0	4.8
Corporate, Non-Segment [Member] | Intersegment revenue
Segment Reporting Information [Line Items]
Revenues	1.2		1.3		2.4	2.5
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues	(1.4)		(1.5)		(2.9)	(2.9)
Depreciation and amortization	0.0		0.0		0.0	0.0
Interest expense	0.0		0.1		0.0	0.2
Net purchased power	(0.2)		(0.3)		(0.5)	(0.5)
Operation and maintenance	(1.2)		(1.2)		(2.4)	(2.4)
Taxes other than income taxes	0.0		0.0		0.0	0.0
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest	0.0		0.0		0.0	0.0
Income tax expense	0.0		0.0		0.0	0.0
Net Income (Loss) Attributable to Parent			0.0			0.0
Segment Reporting, Other Segment Item, Amount	0.0		(0.1)		0.0	(0.2)
Segment, Expenditure, Addition to Long-Lived Assets	0.0		0.0		0.0	0.0
Net income	0.0				0.0
Intersegment Eliminations | Revenue from external customers
Segment Reporting Information [Line Items]
Revenues	0.0		0.0		0.0	0.0
Intersegment Eliminations | Intersegment revenue
Segment Reporting Information [Line Items]
Revenues	(1.4)		$ (1.5)		(2.9)	$ (2.9)
Subsidiaries [Member]
Segment Reporting Information [Line Items]
Total assets	$ 3,462.6				$ 3,462.6		$ 3,355.2