Segment Information (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2019		Sep. 30, 2018		Sep. 30, 2019		Sep. 30, 2018
Segment Reporting Information [Line Items]
Net income	$ 204.5		$ 207.1		$ 444.8		$ 463.1
Gas	153.9		157.2		830.7		707.8
Unregulated Operating Revenue	9.7		8.7		31.8		28.7
Regulated and Unregulated Operating Revenue	1,044.3		1,060.7		3,177.3		3,045.8
Affiliate Revenue	1.1		1.1		3.3		3.3
Regulated Electric
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues	880.8		894.8		2,315.1		2,309.5
Net income	204.7		191.7		371.9		373.3
Regulated Natural Gas
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues	154.0		157.5		830.9		708.2
Net income	0.7		15.1		77.5		89.7
All Other
Segment Reporting Information [Line Items]
Net income	(0.9)		0.3		(4.6)		0.1
Total revenues
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Electric	880.7		894.8		2,314.8		2,309.3
Gas	153.9		157.2		830.7		707.8
Unregulated Operating Revenue	9.7	[1]	8.7	[1]	31.8	[2]	28.7	[2]
Regulated and Unregulated Operating Revenue	1,044.5	[1]	1,061.0	[1]	3,177.8	[2]	3,046.4	[2]
Intersegment Eliminations
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Electric	0.1		0.0		0.3		0.2
Gas	0.1		0.3		0.2		0.4
Regulated and Unregulated Operating Revenue	$ (0.2)		$ (0.3)		$ (0.5)		$ (0.6)

[1] Operating revenues include $1.1 million of other affiliate revenue for the three months ended Sept. 30, 2019 and 2018.

[2] Operating revenues include $3.3 million of other affiliate revenue for the nine months ended Sept. 30, 2019 and 2018.