Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Segment Reporting Information [Line Items]
Regulated Operating Revenue	$ 229.0	$ 232.0	$ 505.5	$ 505.4
Revenues	228.9	231.8	505.1	504.9
Net income	8.6	15.2	32.6	46.6
Affiliate Revenue	43.5	38.4	87.6	76.1
Regulated Electric
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues	208.0	209.6	423.1	426.1
Net income	10.9	15.5	24.1	35.9
Regulated Natural Gas
Segment Reporting Information [Line Items]
Revenues Including Intersegment Revenues	20.9	22.1	82.2	78.8
Net income	(1.3)	(0.1)	9.2	10.3
All Other
Segment Reporting Information [Line Items]
Net income	(1.0)	(0.2)	(0.7)	0.4
Total revenues
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Electric	207.9	209.5	422.9	425.9
Regulated Operating Revenue, Gas	20.9	22.0	82.0	78.5
Regulated Operating Revenue, Other	0.1	0.3	0.2	0.5
Revenues	228.9	231.8	505.1	504.9
Total revenues | Regulated Electric
Segment Reporting Information [Line Items]
Revenues	207.9	209.5	422.9	425.9
Total revenues | Regulated Natural Gas
Segment Reporting Information [Line Items]
Revenues	20.9	22.0	82.0	78.5
Total revenues | All Other
Segment Reporting Information [Line Items]
Revenues	0.1	0.3	0.2	0.5
Intersegment Eliminations
Segment Reporting Information [Line Items]
Regulated Operating Revenue, Electric	0.1	0.1	0.2	0.2
Regulated Operating Revenue, Gas	0.0	0.1	0.2	0.3
Regulated Operating Revenue	$ (0.1)	$ (0.2)	$ (0.4)	$ (0.5)