Business Segments (Narrative) (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) mi² customer segment	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Segment Reporting Information [Line Items]
Total assets	$ 1,907.5		$ 1,907.5		$ 1,935.8
Gain (Loss) on Extinguishment of Debt	(31.7)	$ 0.0	(31.7)	$ (44.9)
Subsidiaries [Member]
Segment Reporting Information [Line Items]
Total assets	1,890.5		$ 1,890.5		1,883.2
Number of Operating Segments | segment			1
Approximate number of retail customers | customer			530,000
Service area, square miles | mi²			6,000
Operating Segments [Member] | Utility [Member]
Segment Reporting Information [Line Items]
Total assets	1,890.5		$ 1,890.5		1,883.2
Gain (Loss) on Extinguishment of Debt	0.0		0.0	0.0
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total assets	17.0		17.0		$ 52.6
Gain (Loss) on Extinguishment of Debt	$ (31.7)		$ (31.7)	$ (44.9)