Revenue (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2017
Revenue from Contract with Customer, Excluding Assessed Tax	$ 168.5	$ 365.0
RTO Revenue	10.6	21.8
RTO Capacity Revenue	4.0	6.8
Revenues	178.1	383.8
Contract with Customer, Asset, Gross	59.4	59.4	$ 63.0
Corporate, Non-Segment [Member]
RTO Revenue	0.1	0.2
RTO Capacity Revenue	2.1	3.0
Revenues	10.3	20.8
Adjustments and Eliminations [Member]
RTO Revenue	0.0	0.0
RTO Capacity Revenue	0.0	0.0
Revenues	(1.0)	(1.8)
Utility [Member]
RTO Revenue	10.5	21.6
RTO Capacity Revenue	1.9	3.8
Revenues	168.8	364.8
Subsidiaries [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	159.2	346.0
Alternative Revenue Programs Accounts Receivable, Gross	3.0	3.0
RTO Revenue	10.5	21.6
RTO Capacity Revenue	1.9	3.8
Revenues	168.8	364.8
Contract with Customer, Asset, Gross	57.2	57.2	$ 62.1
Other Revenues [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	2.7	5.1
Other non-606 revenue	0.0	0.0
Other Revenues [Member] | Corporate, Non-Segment [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	2.7	5.1
Other non-606 revenue	0.8	1.4
Other Revenues [Member] | Adjustments and Eliminations [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0
Other non-606 revenue	(0.8)	(1.4)
Other Revenues [Member] | Utility [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0
Other non-606 revenue	0.0	0.0
Wholesale Revenue [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	11.7	30.8
Wholesale Revenue [Member] | Corporate, Non-Segment [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	4.6	11.1
Wholesale Revenue [Member] | Adjustments and Eliminations [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0
Wholesale Revenue [Member] | Utility [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	7.1	19.7
Wholesale Revenue [Member] | Subsidiaries [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	7.1	19.7
Retail Revenue [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	139.5	300.5
Other non-606 revenue	9.6	18.8
Retail Revenue [Member] | Corporate, Non-Segment [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0
Other non-606 revenue	0.0	0.0
Retail Revenue [Member] | Adjustments and Eliminations [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	(0.2)	(0.4)
Other non-606 revenue	0.0	0.0
Retail Revenue [Member] | Utility [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	139.7	300.9
Other non-606 revenue	9.6	18.8
Retail Revenue [Member] | Subsidiaries [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	139.7	300.9
Other non-606 revenue	$ 9.6	$ 18.8