Business Segments (Narrative) (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2024 USD ($)	Jun. 30, 2023 USD ($)	Jun. 30, 2024 USD ($) mi² segment customer	Jun. 30, 2023 USD ($)	Dec. 31, 2023 USD ($)
Segment Reporting Information [Line Items]
Total assets	$ 3,133.7		$ 3,133.7		$ 2,906.2
Revenues	209.2	$ 202.0	435.6	$ 442.1
Revenue from external customers
Segment Reporting Information [Line Items]
Revenues	209.2	202.0	435.6	442.1
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	3,084.3		$ 3,084.3		2,871.0
Revenues	206.9	199.7	431.2	437.5
Utility [Member]
Segment Reporting Information [Line Items]
Revenues	206.9	199.7	431.2	437.5
Corporate and Other
Segment Reporting Information [Line Items]
Revenues	3.8	3.3	7.3	6.6
Operating Segments [Member] | Utility [Member]
Segment Reporting Information [Line Items]
Revenues	206.9	199.7	431.2	437.5
Operating Segments [Member] | Utility [Member] | Revenue from external customers
Segment Reporting Information [Line Items]
Revenues	206.7	199.5	430.8	437.2
Operating Segments [Member] | Utility [Member] | Intersegment revenue
Segment Reporting Information [Line Items]
Revenues	0.2	0.2	0.4	0.3
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total assets	49.4		49.4		$ 35.2
Revenues	3.8	3.3	7.3	6.6
Corporate, Non-Segment [Member] | Revenue from external customers
Segment Reporting Information [Line Items]
Revenues	2.5	2.5	4.8	4.9
Corporate, Non-Segment [Member] | Intersegment revenue
Segment Reporting Information [Line Items]
Revenues	1.3	0.8	2.5	1.7
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues	(1.5)	(1.0)	(2.9)	(2.0)
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	$ (1.5)	$ (1.0)	$ (2.9)	$ (2.0)