Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Revenue from Contract with Customer, Excluding Assessed Tax	$ 195.0	$ 195.1	$ 576.5	$ 560.1
Alternative Revenue Programs Accounts Receivable, Gross	0.0		0.0		$ 0.0
RTO Revenue	11.1	10.7	33.0	32.5
RTO Capacity Revenue	2.2	3.7	9.2	10.5
Revenues	199.0	207.7	592.2	591.5
Contract with Customer, Asset, Gross	66.3		66.3		72.6
Corporate, Non-Segment [Member]
RTO Revenue	0.1	(0.1)	0.2	0.1
RTO Capacity Revenue	1.1	1.7	4.2	4.7
Revenues	8.9	9.9	25.2	30.7
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	(1.0)	(0.9)	(3.2)	(2.7)
Utility [Member]
RTO Revenue	11.0	10.8	32.8	32.4
RTO Capacity Revenue	1.1	2.0	5.0	5.8
Revenues	191.1	198.7	570.2	563.5
Subsidiaries [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	187.0	186.1	554.4	532.1
Alternative Revenue Programs Accounts Receivable, Gross	0.0		0.0		0.0
RTO Revenue	11.0	10.8	32.8	32.4
RTO Capacity Revenue	1.1	2.0	5.0	5.8
Revenues	191.1	198.7	570.2	563.5
Contract with Customer, Asset, Gross	65.2		65.2		$ 70.1
Other Revenues [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	2.0	2.2	7.1	7.3
Other non-606 revenue	0.2	0.0	0.8	0.0
Other Revenues [Member] | Corporate, Non-Segment [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	2.0	2.2	7.1	7.3
Other non-606 revenue	0.7	0.5	2.3	1.9
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	(0.8)	(0.5)	(2.4)	(1.9)
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	0.3	0.0	0.9	0.0
Other Revenues [Member] | Subsidiaries [Member]
Other non-606 revenue	0.3	0.0	0.9	0.0
Wholesale Revenue [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	9.2	10.5	23.4	41.3
Wholesale Revenue [Member] | Corporate, Non-Segment [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	5.0	5.6	11.4	16.7
Wholesale Revenue [Member] | Adjustments and Eliminations [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	(0.2)	0.0	(0.8)	0.0
Wholesale Revenue [Member] | Utility [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	4.4	4.9	12.8	24.6
Wholesale Revenue [Member] | Subsidiaries [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	4.4	4.9	12.8	24.6
Retail Revenue [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	170.5	168.0	503.8	468.5
Other non-606 revenue	3.8	12.6	14.9	31.4
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] | Adjustments and Eliminations [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	(0.4)	0.0	(0.8)
Other non-606 revenue	0.0	0.0	0.0	0.0
Retail Revenue [Member] | Utility [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	170.5	168.4	503.8	469.3
Other non-606 revenue	3.8	12.6	14.9	31.4
Retail Revenue [Member] | Subsidiaries [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	170.5	168.4	503.8	469.3
Other non-606 revenue	$ 3.8	$ 12.6	$ 14.9	$ 31.4