Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Revenue from Contract with Customer, Excluding Assessed Tax	$ 173.2	$ 168.5	$ 381.5	$ 365.0
Alternative Revenue Programs Accounts Receivable, Gross	0.0		0.0		$ 0.0
RTO Revenue	11.0	10.6	21.9	21.8
RTO Capacity Revenue	3.3	4.0	7.0	6.8
Revenues	184.2	178.1	393.2	383.8
Contract with Customer, Asset, Gross	63.5		63.5		72.6
Corporate, Non-Segment [Member]
RTO Revenue	0.1	0.1	0.1	0.2
RTO Capacity Revenue	1.5	2.1	3.1	3.0
Revenues	7.6	10.3	16.3	20.8
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	(1.1)	(1.0)	(2.2)	(1.8)
Utility [Member]
RTO Revenue	10.9	10.5	21.8	21.6
RTO Capacity Revenue	1.8	1.9	3.9	3.8
Revenues	177.7	168.8	379.1	364.8
Subsidiaries [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	166.7	159.2	367.4	346.0
Alternative Revenue Programs Accounts Receivable, Gross	0.0		0.0		0.0
RTO Revenue	10.9	10.5	21.8	21.6
RTO Capacity Revenue	1.8	1.9	3.9	3.8
Revenues	177.7	168.8	379.1	364.8
Contract with Customer, Asset, Gross	60.7		60.7		$ 70.1
Other Revenues [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	2.2	2.7	5.1	5.1
Other non-606 revenue	0.6	0.0	0.6	0.0
Other Revenues [Member] | Corporate, Non-Segment [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	2.2	2.7	5.1	5.1
Other non-606 revenue	0.8	0.8	1.6	1.4
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)	(0.8)	(1.6)	(1.4)
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	0.6	0.0	0.6	0.0
Other Revenues [Member] | Subsidiaries [Member]
Other non-606 revenue	0.6	0.0	0.6	0.0
Wholesale Revenue [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	5.8	11.7	14.2	30.8
Wholesale Revenue [Member] | Corporate, Non-Segment [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	3.0	4.6	6.4	11.1
Wholesale Revenue [Member] | Adjustments and Eliminations [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	(0.6)	0.0	(0.6)	0.0
Wholesale Revenue [Member] | Utility [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	3.4	7.1	8.4	19.7
Wholesale Revenue [Member] | Subsidiaries [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	3.4	7.1	8.4	19.7
Retail Revenue [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	150.9	139.5	333.3	300.5
Other non-606 revenue	10.4	9.6	11.1	18.8
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	0.3	(0.2)	0.0	(0.4)
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	150.6	139.7	333.3	300.9
Other non-606 revenue	10.4	9.6	11.1	18.8
Retail Revenue [Member] | Subsidiaries [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	150.6	139.7	333.3	300.9
Other non-606 revenue	$ 10.4	$ 9.6	$ 11.1	$ 18.8