Business Segments (Narrative) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2024 USD ($) mi² customer segment	Mar. 31, 2023 USD ($)	Dec. 31, 2023 USD ($)
Segment Reporting Information [Line Items]
Total assets	$ 2,987.7		$ 2,906.2
Revenues	226.4	$ 240.1
Revenue from external customers
Segment Reporting Information [Line Items]
Revenues	226.4	240.1
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	540,000
Service area, square miles | mi²	6,000
Total assets	$ 2,955.9		2,871.0
Revenues	224.3	237.8
Utility [Member]
Segment Reporting Information [Line Items]
Revenues	224.3	237.8
Corporate and Other
Segment Reporting Information [Line Items]
Revenues	3.5	3.3
Operating Segments [Member] | Utility [Member]
Segment Reporting Information [Line Items]
Revenues	224.3	237.8
Operating Segments [Member] | Utility [Member] | Revenue from external customers
Segment Reporting Information [Line Items]
Revenues	224.1	237.7
Operating Segments [Member] | Utility [Member] | Intersegment revenue
Segment Reporting Information [Line Items]
Revenues	0.2	0.1
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total assets	31.8		$ 35.2
Revenues	3.5	3.3
Corporate, Non-Segment [Member] | Revenue from external customers
Segment Reporting Information [Line Items]
Revenues	2.3	2.4
Corporate, Non-Segment [Member] | Intersegment revenue
Segment Reporting Information [Line Items]
Revenues	1.2	0.9
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues	(1.4)	(1.0)
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	$ (1.4)	$ (1.0)