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	$ 29.4		$ 31.0		$ 61.9		$ 77.6
Natural gas	14.6		15.4		96.6		93.8
Unregulated Operating Revenue	0.1		0.3		0.3		0.9
Regulated and Unregulated Operating Revenue	243.7		256.0		748.8		760.9
Affiliate Revenue	44.7		39.9		132.3		116.0
Regulated Electric
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues	229.0		240.4		652.1		666.5
Net income	33.3		34.3		57.4		70.2
Regulated Natural Gas
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues	14.7		15.4		96.9		94.0
Net income	(4.1)		(3.5)		5.1		6.8
All Other
Segment Reporting Information [Line Items]
Net income	0.2		0.2		(0.6)		0.6
Total revenues
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Electric	229.0	[1]	240.3	[1]	651.9	[2]	666.2	[2]
Natural gas	14.6		15.4		96.6		93.8
Unregulated Operating Revenue	0.1		0.3		0.3		0.9
Regulated and Unregulated Operating Revenue	243.8	[1]	256.1	[1]	749.3	[2]	761.4	[2]
Intersegment Eliminations
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Electric	0.0		0.1		0.2		0.3
Natural gas	0.1		0.0		0.3		0.2
Regulated and Unregulated Operating Revenue	$ (0.1)		$ (0.1)		$ (0.5)		$ (0.5)

[1] Operating revenues include $44.7 million and $39.9 million of affiliate electric revenue for the three months ended Sept. 30, 2019 and 2018, respectively.

[2] Operating revenues include $132.3 million and $116.0 million of affiliate electric revenue for the nine months ended Sept. 30 2019 and 2018, respectively.