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	$ 108.1		$ 101.5		$ 236.8		$ 240.3
Regulated Operating Revenue, Electric	715.4		692.7		1,427.2		1,434.2
Regulated Operating Revenue, Gas	186.2		207.6		517.7		676.7
Unregulated Operating Revenue	9.5		9.7		22.9		22.1
Regulated and Unregulated Operating Revenue	911.1		910.0		1,967.8		2,133.0
Related Party Transaction - Electric and Gas Domestic Regulated Revenue	1.1		1.1		2.2		2.2
Electric
Segment Reporting Information [Line Items]
Net income	89.0		84.8		163.0		167.2
Revenues Including Intersegment Revenues	715.5		692.8		1,427.4		1,434.4
Natural gas
Segment Reporting Information [Line Items]
Net income	16.0		17.6		68.1		76.8
Revenues Including Intersegment Revenues					517.6		676.8
All Other
Segment Reporting Information [Line Items]
Net income	3.1		(0.9)		5.7		(3.7)
Total revenues
Segment Reporting Information [Line Items]
Regulated and Unregulated Operating Revenue	911.2	[1]	910.1	[1]	1,967.9	[2]	2,133.3	[2]
Total revenues | Electric
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Electric	715.4		692.7		1,427.2		1,434.2
Total revenues | Natural gas
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Gas	186.2		207.6		517.7		676.7
Total revenues | All Other
Segment Reporting Information [Line Items]
Unregulated Operating Revenue	9.5	[1]	9.7	[1]	22.9	[2]	22.1	[2]
Intersegment Eliminations
Segment Reporting Information [Line Items]
Regulated and Unregulated Operating Revenue	(0.1)		(0.1)		(0.1)		(0.3)
Intersegment Eliminations | Electric
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Electric	$ 0.1		$ 0.1		0.2		0.2
Intersegment Eliminations | Natural gas
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Gas					$ (0.1)		$ 0.1

[1] Operating revenues include $1.1 million of other affiliate revenue for the three months ended June 30, 2020 and 2019.

[2] Operating revenues include $2.2 million of other affiliate revenue for the six months ended June 30, 2020 and 2019.