Business Segments (Narrative) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2023 USD ($) mi² segment customer	Dec. 31, 2022 USD ($)
Segment Reporting Information [Line Items]
Total assets	$ 2,467.1	$ 2,422.4
Subsidiaries [Member]
Segment Reporting Information [Line Items]
Number of Operating Segments | segment	1
Approximate number of retail customers | customer	537,000
Service area, square miles | mi²	6,000
Total assets	$ 2,446.5	2,405.9
Operating Segments [Member] | Utility [Member]
Segment Reporting Information [Line Items]
Total assets		2,405.9
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total assets	$ 20.6	$ 16.5