Business Segments (Narrative) (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) mi² customer segment	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Segment Reporting Information [Line Items]
Total assets	$ 2,048.9		$ 2,048.9		$ 2,036.0
Revenue from Contract with Customer, Excluding Assessed Tax	147.4	$ 148.3	319.5	$ 315.3
RTO Revenue	12.1	10.2	27.0	20.9
RTO Capacity Revenue	1.3	1.1	2.2	2.3
Revenues	148.1	155.1	323.3	326.3
Retail Revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	128.3	133.3	277.3	283.5
Other non-606 revenue		5.4		8.7
Wholesale Revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	3.4	1.5	8.2	4.0
Other Revenues [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	2.3	2.2	4.8	4.6
Other non-606 revenue	0.7	1.4	3.8	2.3
Residential Revenue | Retail Revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	77.7	83.7	176.0	180.6
Commercial Revenue | Retail Revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	28.2	26.5	55.5	55.4
Industrial Revenue | Retail Revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	14.1	11.9	26.4	23.7
Governmental Revenue | Retail Revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	5.5	8.6	12.9	17.8
Other Revenues [Member] | Retail Revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	2.8	2.6	6.5	6.0
Subsidiaries [Member]
Segment Reporting Information [Line Items]
Total assets	2,025.5		$ 2,025.5		2,014.7
Number of Operating Segments | segment			1
Approximate number of retail customers | customer			532,000
Service area, square miles | mi²			6,000
Revenue from Contract with Customer, Excluding Assessed Tax	145.3	146.3	$ 315.0	311.1
RTO Revenue	12.1	10.1	26.9	20.8
RTO Capacity Revenue	1.3	1.1	2.2	2.3
Revenues	145.9	153.1	318.7	322.1
Subsidiaries [Member] | Wholesale Revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	3.6	1.8	8.6	4.5
Subsidiaries [Member] | Other Revenues [Member]
Segment Reporting Information [Line Items]
Other non-606 revenue	0.6	1.4	3.7	2.3
Utility [Member]
Segment Reporting Information [Line Items]
RTO Revenue	12.1	10.1	26.9	20.8
RTO Capacity Revenue	1.3	1.1	2.2	2.3
Revenues	145.9	153.1	318.7	322.1
Utility [Member] | Retail Revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	128.3	133.3	277.3	283.5
Other non-606 revenue	0.0	5.4	0.0	8.7
Utility [Member] | Wholesale Revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	3.6	1.8	8.6	4.5
Utility [Member] | Other Revenues [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Other non-606 revenue	0.6	1.4	3.7	2.3
Utility [Member] | Residential Revenue | Retail Revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	77.7	83.7	176.0	180.6
Utility [Member] | Commercial Revenue | Retail Revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	28.2	26.5	55.5	55.4
Utility [Member] | Industrial Revenue | Retail Revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	14.1	11.9	26.4	23.7
Utility [Member] | Governmental Revenue | Retail Revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	5.5	8.6	12.9	17.8
Utility [Member] | Other Revenues [Member] | Retail Revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	2.8	2.6	6.5	6.0
Corporate and Other
Segment Reporting Information [Line Items]
RTO Revenue	0.0	0.1	0.1	0.1
RTO Capacity Revenue	0.0	0.0	0.0	0.0
Revenues	3.3	3.2	6.8	6.4
Corporate and Other | Retail Revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Other non-606 revenue		0.0		0.0
Corporate and Other | Wholesale Revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Corporate and Other | Other Revenues [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	2.3	2.2	4.8	4.6
Other non-606 revenue	1.0	0.9	1.9	1.7
Corporate and Other | Residential Revenue | Retail Revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Corporate and Other | Commercial Revenue | Retail Revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Corporate and Other | Industrial Revenue | Retail Revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Corporate and Other | Governmental Revenue | Retail Revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Corporate and Other | Other Revenues [Member] | Retail Revenue [Member]
Segment Reporting Information [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	$ 0.0	0.0	$ 0.0
Operating Segments [Member] | Utility [Member]
Segment Reporting Information [Line Items]
Total assets					2,014.7
Corporate, Non-Segment [Member]
Segment Reporting Information [Line Items]
Total assets	$ 23.4		$ 23.4		$ 21.3