Business Segments (Narrative) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2025 USD ($) mi² segment customer	Mar. 31, 2024 USD ($)	Dec. 31, 2024 USD ($)
Segment Reporting Information [Line Items]
Number of Operating Segments | segment	1
Total assets	$ 3,489.4		$ 3,417.4
Revenues	254.3	$ 226.4
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
Subsidiaries [Member]
Segment Reporting Information [Line Items]
Number of Operating Segments | segment	1
Approximate number of retail customers | customer	543,000
Service area, square miles | mi²	6,000
Total assets	$ 3,432.6		3,355.2
Revenues	252.1	224.3
Utility [Member]
Segment Reporting Information [Line Items]
Revenues	252.1	224.3
Corporate and Other
Segment Reporting Information [Line Items]
Revenues	3.7	3.5
Operating Segments [Member] | Utility [Member]
Segment Reporting Information [Line Items]
Revenues	252.1	224.3
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]
Total assets	56.8		$ 62.2
Revenues	3.7	3.5
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)
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)