Segment Information (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2020		Sep. 30, 2019		Sep. 30, 2020		Sep. 30, 2019
Segment Reporting Information [Line Items]
Net Income (Loss) Attributable to Parent	$ 39.5		$ 29.4		$ 85.2		$ 61.9
Natural gas	14.5		14.6		78.7		96.6
Unregulated Operating Revenue	0.1		0.1		0.3		0.3
Regulated and Unregulated Operating Revenue	250.5		243.7		726.1		748.8
Affiliate Revenue	41.0		44.7		123.5		132.3
Electricity, US Regulated [Member]
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues					647.3		652.1
Net Income (Loss) Attributable to Parent	39.8		33.3		80.8		57.4
Natural Gas, US Regulated [Member]
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues	14.6		14.7		78.9		96.9
Net Income (Loss) Attributable to Parent	(2.4)		(4.1)		3.6		5.1
All Other
Segment Reporting Information [Line Items]
Net Income (Loss) Attributable to Parent	2.1		0.2		0.8		(0.6)
Total revenues
Segment Reporting Information [Line Items]
Regulated and Unregulated Operating Revenue	250.6	[1]	243.8	[1]	726.5	[2]	749.3	[2]
Total revenues | Electricity, US Regulated [Member]
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Electric	235.9	[1]	229.0	[1]	647.1	[2]	651.9	[2]
Total revenues | Natural Gas, US Regulated [Member]
Segment Reporting Information [Line Items]
Natural gas	14.5		14.6		78.7		96.6
Total revenues | All Other
Segment Reporting Information [Line Items]
Unregulated Operating Revenue	0.1		0.1		0.3		0.3
Intersegment Eliminations
Segment Reporting Information [Line Items]
Regulated and Unregulated Operating Revenue	(0.1)		(0.1)		(0.4)		(0.5)
Intersegment Eliminations | Electricity, US Regulated [Member]
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Electric					0.2		0.2
Intersegment Eliminations | Natural Gas, US Regulated [Member]
Segment Reporting Information [Line Items]
Natural gas	$ 0.1		$ 0.1		$ 0.2		$ 0.3

[1] Operating revenues include $41.0 million and $44.7 million of affiliate electric revenue for the three months ended Sept. 30, 2020 and 2019, respectively.

[2] Operating revenues include $123.5 million and $132.3 million of affiliate electric revenue for the nine months ended Sept. 30, 2020 and 2019, respectively