Business Segments (Narrative) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2022 USD ($) mi² segment customer	Dec. 31, 2021 USD ($)
Segment Reporting Information [Line Items]
Total assets	$ 2,189.7	$ 2,171.8
Subsidiaries [Member]
Segment Reporting Information [Line Items]
Number of Operating Segments | segment	1
Approximate number of retail customers | customer	535,000
Service area, square miles | mi²	6,000
Total assets	$ 2,175.7	2,162.6
Operating Segments [Member] | Utility [Member]
Segment Reporting Information [Line Items]
Total assets		2,162.6
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total assets	$ 14.0	$ 9.2