Segment Information (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Net income		$ 33.6	$ 24.0
Natural gas		45.2	61.1
Unregulated Operating Revenue		0.0	0.1
Regulated and Unregulated Operating Revenue		257.0	276.2
Affiliate Revenue		40.9	44.1
Electricity, US Regulated [Member]
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues		211.9	215.1
Net income		24.7	13.2
Natural Gas, US Regulated [Member]
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues		45.3	61.3
Net income		8.9	10.5
All Other
Segment Reporting Information [Line Items]
Net income		0.0	0.3
Total revenues
Segment Reporting Information [Line Items]
Regulated and Unregulated Operating Revenue	[1]	257.2	276.5
Total revenues | Electricity, US Regulated [Member]
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Electric	[1]	211.8	215.0
Total revenues | Natural Gas, US Regulated [Member]
Segment Reporting Information [Line Items]
Natural gas		45.2	61.1
Total revenues | All Other
Segment Reporting Information [Line Items]
Unregulated Operating Revenue		0.0	0.1
Intersegment Eliminations
Segment Reporting Information [Line Items]
Regulated and Unregulated Operating Revenue		(0.2)	(0.3)
Intersegment Eliminations | Electricity, US Regulated [Member]
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Electric		0.1	0.1
Intersegment Eliminations | Natural Gas, US Regulated [Member]
Segment Reporting Information [Line Items]
Natural gas		$ 0.1	$ 0.2

[1] Operating revenues include $40.9 million and $44.1 million of affiliate electric revenue for the three months ended March 31, 2020 and 2019, respectively.