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	$ 117.4		$ 95.9		$ 224.4		$ 209.1
Regulated Operating Revenue, Gas	75.0		79.4		281.1		343.5
Unregulated Operating Revenue	9.2		8.4		18.0		16.1
Regulated and Unregulated Operating Revenue	1,179.9		1,185.0		2,430.1		2,535.5
Electricity, US Regulated [Member]
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues	1,095.8		1,097.4		2,131.2		2,176.2
Net income	113.2		93.9		199.2		176.8
Natural Gas, US Regulated [Member]
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues	75.1		79.6		281.4		344.0
Net income	0.6		0.3		23.9		29.0
All Other
Segment Reporting Information [Line Items]
Net income	3.6		1.7		1.3		3.3
Total revenues
Segment Reporting Information [Line Items]
Regulated and Unregulated Operating Revenue	1,180.1	[1],[2]	1,185.4	[1],[2]	2,430.6	[3],[4]	2,536.3	[3],[4]
Total revenues | Electricity, US Regulated [Member]
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Electric	1,095.7	[1]	1,097.2	[1]	2,131.0	[3]	2,175.9	[3]
Total revenues | Natural Gas, US Regulated [Member]
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Gas	75.0	[2]	79.4	[2]	281.1	[4]	343.5	[4]
Total revenues | All Other
Segment Reporting Information [Line Items]
Unregulated Operating Revenue	9.2		8.4		18.0		16.1
Intersegment Eliminations
Segment Reporting Information [Line Items]
Regulated and Unregulated Operating Revenue	(0.2)		(0.4)		(0.5)		(0.8)
Intersegment Eliminations | Electricity, US Regulated [Member]
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Electric	0.1		0.2		0.2		0.3
Intersegment Eliminations | Natural Gas, US Regulated [Member]
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Gas	$ 0.1		$ 0.2		$ 0.3		$ 0.5

[1] Operating revenues include $106.1 million and $116.2 million of affiliate electric revenue for the three months ended June 30, 2020 and 2019.

[2] Operating revenues includes an immaterial amount of affiliate gas revenue for the three months ended June 30, 2020 and 2019.

[3] Operating revenues include $215.7 million and $236.3 million of affiliate electric revenue for the six months ended June 30, 2020 and 2019.

[4] Operating revenues includes an immaterial amount of affiliate gas revenue for the six months ended June 30, 2020 and 2019.