Segment Information (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Segment Reporting Information [Line Items]
Net income		$ 107.0	$ 113.2
Regulated Operating Revenue, Gas		206.1	264.0
Unregulated Operating Revenue		8.9	7.8
Regulated and Unregulated Operating Revenue		1,250.2	1,350.5
Affiliate Revenue		109.6	120.1
Electricity, US Regulated [Member]
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues		1,035.3	1,078.8
Net income		86.0	82.9
Natural Gas, US Regulated [Member]
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues		206.3	264.3
Net income		23.3	28.7
All Other
Segment Reporting Information [Line Items]
Net income		(2.3)	1.6
Total revenues
Segment Reporting Information [Line Items]
Regulated and Unregulated Operating Revenue	[1],[2]	1,250.5	1,350.9
Total revenues | Electricity, US Regulated [Member]
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Electric	[1]	1,035.2	1,078.7
Total revenues | Natural Gas, US Regulated [Member]
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Gas	[2]	206.1	264.0
Total revenues | All Other
Segment Reporting Information [Line Items]
Unregulated Operating Revenue		8.9	7.8
Intersegment Eliminations
Segment Reporting Information [Line Items]
Regulated and Unregulated Operating Revenue		(0.3)	(0.4)
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]
Regulated Operating Revenue, Gas		$ 0.2	$ 0.3

[1] Operating revenues include $109.6 million and $120.1 million of affiliate electric revenue for the three months ended March 31, 2020 and 2019.

[2] Operating revenues includes an immaterial amount of affiliate gas revenue for the three months ended March 31, 2020 and 2019.