Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2021
Revenue from Contract with Customer, Excluding Assessed Tax	$ 197.7	$ 172.1
RTO Revenue	15.1	14.9
RTO Capacity Revenue	1.8	0.9
Revenues	199.3	175.2
Contract with Customer, Asset, before Allowance for Credit Loss	64.4		$ 61.5
Subsidiaries [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	195.4	169.7
RTO Revenue	15.1	14.8
RTO Capacity Revenue	1.8	0.9
Revenues	197.0	172.8
Contract with Customer, Asset, before Allowance for Credit Loss	63.5		$ 60.8
Utility [Member]
RTO Revenue	15.1	14.8
RTO Capacity Revenue	1.8	0.9
Revenues	197.0	172.8
Corporate and Other
RTO Revenue	0.0	0.1
RTO Capacity Revenue	0.0	0.0
Revenues	3.4	3.5
Adjustments and Eliminations
RTO Revenue	0.0	0.0
RTO Capacity Revenue	0.0	0.0
Revenues	(1.1)	(1.1)
Other Revenues [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	2.5	2.5
Other non-606 revenue	1.6	3.1
Other Revenues [Member] | Subsidiaries [Member]
Other non-606 revenue	1.6	3.1
Other Revenues [Member] | Utility [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0
Other non-606 revenue	1.6	3.1
Other Revenues [Member] | Corporate and Other
Revenue from Contract with Customer, Excluding Assessed Tax	2.5	2.5
Other non-606 revenue	0.9	0.9
Other Revenues [Member] | Adjustments and Eliminations
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0
Other non-606 revenue	(0.9)	(0.9)
Wholesale Revenue [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	7.5	4.8
Wholesale Revenue [Member] | Subsidiaries [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	7.7	5.0
Wholesale Revenue [Member] | Utility [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	7.7	5.0
Wholesale Revenue [Member] | Corporate and Other
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0
Wholesale Revenue [Member] | Adjustments and Eliminations
Revenue from Contract with Customer, Excluding Assessed Tax	(0.2)	(0.2)
Retail Revenue [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	170.8	149.0
Retail Revenue [Member] | Residential Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	109.9	98.3
Retail Revenue [Member] | Commercial Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	35.0	27.3
Retail Revenue [Member] | Industrial Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	16.9	12.3
Retail Revenue [Member] | Governmental Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	6.1	7.4
Retail Revenue [Member] | Other Revenues [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	2.9	3.7
Retail Revenue [Member] | Utility [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	170.8	149.0
Retail Revenue [Member] | Utility [Member] | Residential Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	109.9	98.3
Retail Revenue [Member] | Utility [Member] | Commercial Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	35.0	27.3
Retail Revenue [Member] | Utility [Member] | Industrial Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	16.9	12.3
Retail Revenue [Member] | Utility [Member] | Governmental Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	6.1	7.4
Retail Revenue [Member] | Utility [Member] | Other Revenues [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	2.9	3.7
Retail Revenue [Member] | Corporate and Other
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0
Retail Revenue [Member] | Corporate and Other | Residential Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0
Retail Revenue [Member] | Corporate and Other | Commercial Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0
Retail Revenue [Member] | Corporate and Other | Industrial Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0
Retail Revenue [Member] | Corporate and Other | Governmental Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0
Retail Revenue [Member] | Corporate and Other | Other Revenues [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0
Retail Revenue [Member] | Adjustments and Eliminations
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0
Retail Revenue [Member] | Adjustments and Eliminations | Residential Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0
Retail Revenue [Member] | Adjustments and Eliminations | Commercial Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0
Retail Revenue [Member] | Adjustments and Eliminations | Industrial Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0
Retail Revenue [Member] | Adjustments and Eliminations | Governmental Revenue
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0
Retail Revenue [Member] | Adjustments and Eliminations | Other Revenues [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 0.0	$ 0.0