Business Segments (Narrative) (Details) $ in Millions	9 Months Ended
	Sep. 30, 2021 USD ($) mi² customer segment	Dec. 31, 2020 USD ($)
Segment Reporting Information [Line Items]
Total assets	$ 2,088.7	$ 2,036.0
Subsidiaries [Member]
Segment Reporting Information [Line Items]
Number of Operating Segments | segment	1
Approximate number of retail customers | customer	533,000
Service area, square miles | mi²	6,000
Total assets	$ 2,067.6	2,014.7
Operating Segments [Member] | Utility [Member]
Segment Reporting Information [Line Items]
Total assets		2,014.7
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total assets	$ 21.1	$ 21.3