Business Segments (Narrative) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2024 USD ($) mi² customer segment	Sep. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)
Segment Reporting Information [Line Items]
Total assets	$ 3,200.5		$ 3,200.5		$ 2,906.2
Revenues	227.5	$ 224.2	663.1	$ 666.3
Revenue from external customers
Segment Reporting Information [Line Items]
Revenues	227.5	224.2	663.1	666.3
Intersegment revenue
Segment Reporting Information [Line Items]
Revenues	0.0	0.0	$ 0.0	0.0
Subsidiaries [Member]
Segment Reporting Information [Line Items]
Number of Operating Segments | segment			1
Approximate number of retail customers | customer			541,000
Service area, square miles | mi²			6,000
Total assets	3,150.2		$ 3,150.2		2,871.0
Revenues	225.4	222.0	656.6	659.5
Utility [Member]
Segment Reporting Information [Line Items]
Revenues	225.4	222.0	656.6	659.5
Corporate and Other
Segment Reporting Information [Line Items]
Revenues	3.6	3.9	10.9	10.5
Operating Segments [Member] | Utility [Member]
Segment Reporting Information [Line Items]
Revenues	225.4	222.0	656.6	659.5
Operating Segments [Member] | Utility [Member] | Revenue from external customers
Segment Reporting Information [Line Items]
Revenues	225.1	221.8	655.9	659.0
Operating Segments [Member] | Utility [Member] | Intersegment revenue
Segment Reporting Information [Line Items]
Revenues	0.3	0.2	0.7	0.5
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total assets	50.3		50.3		$ 35.2
Revenues	3.6	3.9	10.9	10.5
Corporate, Non-Segment [Member] | Revenue from external customers
Segment Reporting Information [Line Items]
Revenues	2.4	2.4	7.2	7.3
Corporate, Non-Segment [Member] | Intersegment revenue
Segment Reporting Information [Line Items]
Revenues	1.2	1.5	3.7	3.2
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues	(1.5)	(1.7)	(4.4)	(3.7)
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	$ (1.5)	$ (1.7)	$ (4.4)	$ (3.7)