Segment Information (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2020		Sep. 30, 2019		Sep. 30, 2020		Sep. 30, 2019
Segment Reporting Information [Line Items]
Net income	$ 218.4		$ 204.5		$ 455.2		$ 444.8
Regulated Operating Revenue, Electric	915.9		880.7		2,343.1		2,314.8
Regulated Operating Revenue, Gas	156.4		153.9		674.1		830.7
Unregulated Operating Revenue	8.8		9.7		31.7		31.8
Regulated and Unregulated Operating Revenue	1,081.1		1,044.3		3,048.9		3,177.3
Affiliate Revenue	1.1		1.1		3.3		3.3
Electric
Segment Reporting Information [Line Items]
Net income	214.5		204.7		377.5		371.9
Revenues Including Intersegment Revenues	916.0		880.8		2,343.4		2,315.1
Natural gas
Segment Reporting Information [Line Items]
Net income	4.4		0.7		72.5		77.5
Revenues Including Intersegment Revenues	156.5		154.0		674.1		830.9
All Other
Segment Reporting Information [Line Items]
Net income	(0.5)		(0.9)		5.2		(4.6)
Total revenues
Segment Reporting Information [Line Items]
Regulated and Unregulated Operating Revenue	1,081.3	[1]	1,044.5	[1]	3,049.2	[2]	3,177.8	[2]
Total revenues | Electric
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Electric	915.9		880.7		2,343.1		2,314.8
Total revenues | Natural gas
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Gas	156.4		153.9		674.1		830.7
Total revenues | All Other
Segment Reporting Information [Line Items]
Unregulated Operating Revenue	8.8	[3]	9.7	[1]	31.7	[2]	31.8	[2]
Intersegment Eliminations
Segment Reporting Information [Line Items]
Regulated and Unregulated Operating Revenue	(0.2)		(0.2)		(0.3)		(0.5)
Intersegment Eliminations | Electric
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Electric	0.1		0.1		0.3		0.3
Intersegment Eliminations | Natural gas
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Gas	$ 0.1		$ 0.1		$ 0.0		$ 0.2

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

[2] Operating revenues include $3.3 million of other affiliate revenue for the nine months ended Sept. 30, 2020 and 2019.

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