Revenue (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Revenue from Contract with Customer, Excluding Assessed Tax	$ 290.3
Derivatives Accounts Receivable, Gross	1.7
RTO Revenue	12.9
RTO Capacity Revenue	29.2
Revenues	293.2
Contract with Customer, Asset, Gross	73.0	$ 79.4
Transmission and Distribution [Member]
RTO Revenue	11.1
RTO Capacity Revenue	1.9
Revenues	196.0
Generation [Member]
RTO Revenue	1.8
RTO Capacity Revenue	27.3
Revenues	95.0
Corporate, Non-Segment [Member]
RTO Revenue	0.0
RTO Capacity Revenue	0.0
Revenues	3.0
Adjustments and Eliminations [Member]
RTO Revenue	0.0
RTO Capacity Revenue	0.0
Revenues	(0.8)
Subsidiaries [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	186.8
Alternative Revenue Programs Accounts Receivable, Gross	3.1
RTO Revenue	11.1
RTO Capacity Revenue	1.9
Revenues	196.0
Contract with Customer, Asset, Gross	58.8	$ 62.1
Other Revenues [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	2.4
Other non-606 revenue	0.0
Other Revenues [Member] | Transmission and Distribution [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0
Other non-606 revenue	0.0
Other Revenues [Member] | Generation [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0
Other non-606 revenue	0.0
Other Revenues [Member] | Corporate, Non-Segment [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	2.4
Other non-606 revenue	0.6
Other Revenues [Member] | Adjustments and Eliminations [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0
Other non-606 revenue	(0.6)
Wholesale Revenue [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	84.8
Derivative Revenue	(6.3)
Wholesale Revenue [Member] | Transmission and Distribution [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	12.6
Derivative Revenue	0.0
Wholesale Revenue [Member] | Generation [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	72.2
Derivative Revenue	(6.3)
Wholesale Revenue [Member] | Corporate, Non-Segment [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0
Derivative Revenue	0.0
Wholesale Revenue [Member] | Adjustments and Eliminations [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0
Derivative Revenue	0.0
Wholesale Revenue [Member] | Subsidiaries [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	12.6
Retail Revenue [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	161.0
Other non-606 revenue	9.2
Retail Revenue [Member] | Transmission and Distribution [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	161.2
Other non-606 revenue	9.2
Retail Revenue [Member] | Generation [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0
Other non-606 revenue	0.0
Retail Revenue [Member] | Corporate, Non-Segment [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0
Other non-606 revenue	0.0
Retail Revenue [Member] | Adjustments and Eliminations [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	(0.2)
Other non-606 revenue	0.0
Retail Revenue [Member] | Subsidiaries [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	161.2
Other non-606 revenue	$ 9.2