Business Segments (Narrative) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Sep. 30, 2023 USD ($) mi² segment customer	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)
Segment Reporting Information [Line Items]
Total assets	$ 2,655.7		$ 2,655.7		$ 2,422.4
Revenues	224.2	$ 262.1	666.3	$ 652.3
Revenue from external customers
Segment Reporting Information [Line Items]
Revenues	224.2	262.1	666.3	652.3
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			538,000
Service area, square miles | mi²			6,000
Total assets	2,638.0		$ 2,638.0		2,405.9
Revenues	222.0	259.9	659.5	645.6
Utility [Member]
Segment Reporting Information [Line Items]
Revenues	222.0	259.9	659.5	645.6
Corporate and Other
Segment Reporting Information [Line Items]
Revenues	3.9	3.2	10.5	9.9
Operating Segments [Member] | Utility [Member]
Segment Reporting Information [Line Items]
Total assets					2,405.9
Revenues	222.0	259.9	659.5	645.6
Operating Segments [Member] | Utility [Member] | Revenue from external customers
Segment Reporting Information [Line Items]
Revenues	221.8	259.7	659.0	645.0
Operating Segments [Member] | Utility [Member] | Intersegment revenue
Segment Reporting Information [Line Items]
Revenues	0.2	0.2	0.5	0.6
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total assets	17.7		17.7		$ 16.5
Revenues	3.9	3.2	10.5	9.9
Corporate, Non-Segment [Member] | Revenue from external customers
Segment Reporting Information [Line Items]
Revenues	2.4	2.4	7.3	7.3
Corporate, Non-Segment [Member] | Intersegment revenue
Segment Reporting Information [Line Items]
Revenues	1.5	0.8	3.2	2.6
Intersegment Eliminations
Segment Reporting Information [Line Items]
Revenues	(1.7)	(1.0)	(3.7)	(3.2)
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	$ (1.7)	$ (1.0)	$ (3.7)	$ (3.2)