Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Revenue from Contract with Customer, Excluding Assessed Tax	$ 177.0	$ 188.9	$ 492.3	$ 560.8
RTO Revenue	11.4	11.1	32.3	32.9
RTO Capacity Revenue	0.9	1.1	3.2	5.0
Revenues	178.8	192.9	505.1	576.5
Contract with Customer, Asset, before Allowance for Credit Loss	67.3		67.3		$ 65.1
Corporate, Non-Segment [Member]
RTO Revenue	(0.1)	0.1	0.0	0.1
RTO Capacity Revenue	0.0	0.0	0.0	0.0
Revenues	3.0	2.8	9.4	9.5
Adjustments and Eliminations [Member]
RTO Revenue	0.0	0.0	0.0	0.0
RTO Capacity Revenue	0.0	0.0	0.0	0.0
Revenues	(1.2)	(1.0)	(3.4)	(3.2)
Utility [Member]
RTO Revenue	11.5	11.0	32.3	32.8
RTO Capacity Revenue	0.9	1.1	3.2	5.0
Revenues	177.0	191.1	499.1	570.2
Subsidiaries [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	175.2	187.0	486.3	554.4
RTO Revenue	11.5	11.0	32.3	32.8
RTO Capacity Revenue	0.9	1.1	3.2	5.0
Revenues	177.0	191.1	499.1	570.2
Contract with Customer, Asset, before Allowance for Credit Loss	66.3		66.3		$ 64.4
Other Revenues [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	2.1	2.0	6.7	7.1
Other non-606 revenue	1.7	0.2	4.0	0.8
Other Revenues [Member] | Corporate, Non-Segment [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	2.1	2.0	6.7	7.1
Other non-606 revenue	1.0	0.7	2.7	2.3
Other Revenues [Member] | Adjustments and Eliminations [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Other non-606 revenue	(1.0)	(0.8)	(2.7)	(2.4)
Other Revenues [Member] | Utility [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Other non-606 revenue	1.7	0.3	4.0	0.9
Other Revenues [Member] | Subsidiaries [Member]
Other non-606 revenue	1.7	0.3	4.0	0.9
Wholesale Revenue [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	3.0	4.2	7.0	12.0
Wholesale Revenue [Member] | Corporate, Non-Segment [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Wholesale Revenue [Member] | Adjustments and Eliminations [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	(0.2)	(0.2)	(0.7)	(0.8)
Wholesale Revenue [Member] | Utility [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	3.2	4.4	7.7	12.8
Wholesale Revenue [Member] | Subsidiaries [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	3.2	4.4	7.7	12.8
Retail Revenue [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	159.6	170.5	443.1	503.8
Other non-606 revenue	0.1	3.8	8.8	14.9
Retail Revenue [Member] | Residential Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	99.3	107.5	279.9	313.9
Retail Revenue [Member] | Commercial Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	32.2	34.8	87.6	103.7
Retail Revenue [Member] | Industrial Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	14.5	13.7	38.2	43.5
Retail Revenue [Member] | Governmental Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	9.9	11.4	27.7	33.3
Retail Revenue [Member] | Other Revenues [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	3.7	3.1	9.7	9.4
Retail Revenue [Member] | Corporate, Non-Segment [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Other non-606 revenue	0.0	0.0	0.0	0.0
Retail Revenue [Member] | Corporate, Non-Segment [Member] | Residential Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Retail Revenue [Member] | Corporate, Non-Segment [Member] | Commercial Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Retail Revenue [Member] | Corporate, Non-Segment [Member] | Industrial Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Retail Revenue [Member] | Corporate, Non-Segment [Member] | Governmental Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Retail Revenue [Member] | Corporate, Non-Segment [Member] | Other Revenues [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Retail Revenue [Member] | Adjustments and Eliminations [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Other non-606 revenue	0.0	0.0	0.0	0.0
Retail Revenue [Member] | Adjustments and Eliminations [Member] | Residential Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Retail Revenue [Member] | Adjustments and Eliminations [Member] | Commercial Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Retail Revenue [Member] | Adjustments and Eliminations [Member] | Industrial Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Retail Revenue [Member] | Adjustments and Eliminations [Member] | Governmental Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Retail Revenue [Member] | Adjustments and Eliminations [Member] | Other Revenues [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Retail Revenue [Member] | Utility [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	159.6	170.5	443.1	503.8
Other non-606 revenue	0.1	3.8	8.8	14.9
Retail Revenue [Member] | Utility [Member] | Residential Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	99.3	107.5	279.9	313.9
Retail Revenue [Member] | Utility [Member] | Commercial Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	32.2	34.8	87.6	103.7
Retail Revenue [Member] | Utility [Member] | Industrial Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	14.5	13.7	38.2	43.5
Retail Revenue [Member] | Utility [Member] | Governmental Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	9.9	11.4	27.7	33.3
Retail Revenue [Member] | Utility [Member] | Other Revenues [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 3.7	$ 3.1	$ 9.7	$ 9.4