Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Revenue from Contract with Customer, Excluding Assessed Tax	$ 208.3	$ 198.2
Alternative Revenue Programs Accounts Receivable, Gross	1.5		$ 0.0
RTO Revenue	10.9	11.2
RTO Capacity Revenue	3.7	3.3
Revenues	209.0	207.4
Contract with Customer, Asset, Gross	74.8		72.6
Corporate, Non-Segment [Member]
RTO Revenue	0.0	0.1
RTO Capacity Revenue	1.6	1.4
Revenues	8.7	12.2
Adjustments and Eliminations [Member]
RTO Revenue	0.0	0.0
RTO Capacity Revenue	0.0	0.0
Revenues	(1.1)	(0.8)
Utility [Member]
RTO Revenue	10.9	11.1
RTO Capacity Revenue	2.1	1.9
Revenues	201.4	196.0
Subsidiaries [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	200.7	186.8
Alternative Revenue Programs Accounts Receivable, Gross	1.5		0.0
RTO Revenue	10.9	11.1
RTO Capacity Revenue	2.1	1.9
Revenues	201.4	196.0
Contract with Customer, Asset, Gross	72.4		$ 70.1
Other Revenues [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	2.9	2.4
Other non-606 revenue	0.0	0.0
Other Revenues [Member] | Corporate, Non-Segment [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	2.9	2.4
Other non-606 revenue	0.8	0.6
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)	(0.6)
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	8.4	20.3
Wholesale Revenue [Member] | Corporate, Non-Segment [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	3.4	7.7
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	5.0	12.6
Wholesale Revenue [Member] | Subsidiaries [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	5.0	12.6
Retail Revenue [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	182.4	161.0
Other non-606 revenue	0.7	9.2
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	(0.3)	(0.2)
Other non-606 revenue	0.0	0.0
Retail Revenue [Member] | Utility [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	182.7	161.2
Other non-606 revenue	0.7	9.2
Retail Revenue [Member] | Subsidiaries [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	182.7	161.2
Other non-606 revenue	$ 0.7	$ 9.2