Segment Information (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Segment Reporting Information [Line Items]
Net income		$ 12.1	$ 8.6	$ 45.7	$ 32.6
Natural gas		19.0	20.9	64.2	82.0
Unregulated Operating Revenue		0.2	0.1	0.2	0.2
Regulated and Unregulated Operating Revenue		218.5	228.9	475.5	505.1
Affiliate Revenue		41.5	43.5	82.4	87.6
Electricity, US Regulated [Member]
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues		199.4	208.0	411.3	423.1
Net income		16.3	10.9	41.0	24.1
Natural Gas, US Regulated [Member]
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues		19.2	20.9	64.5	82.2
Net income		(2.9)	(1.3)	6.0	9.2
All Other
Segment Reporting Information [Line Items]
Net income		(1.3)	(1.0)	(1.3)	(0.7)
Total revenues
Segment Reporting Information [Line Items]
Regulated and Unregulated Operating Revenue	[1]	218.8	229.0	476.0	505.5
Total revenues | Electricity, US Regulated [Member]
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Electric	[1]	199.3	207.9	411.1	422.9
Total revenues | Natural Gas, US Regulated [Member]
Segment Reporting Information [Line Items]
Natural gas		19.0	20.9	64.2	82.0
Total revenues | All Other
Segment Reporting Information [Line Items]
Unregulated Operating Revenue		0.2	0.1	0.2	0.2
Intersegment Eliminations
Segment Reporting Information [Line Items]
Regulated and Unregulated Operating Revenue		(0.3)	(0.1)	(0.5)	(0.4)
Intersegment Eliminations | Electricity, US Regulated [Member]
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Electric		0.1	0.1	0.2	0.2
Intersegment Eliminations | Natural Gas, US Regulated [Member]
Segment Reporting Information [Line Items]
Natural gas		$ 0.2	$ 0.0	$ 0.3	$ 0.2

[1] Operating revenues include $41.5 million and $43.5 million of affiliate electric revenue for the three months ended June 30, 2020 and 2019, respectively.