Revenues (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	$ 257.0	$ 276.2
Regulated Electric \| Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	211.8	215.0
Regulated Natural Gas \| Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	45.2	61.1
All Other \| Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	0.0	0.1
Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	253.2	272.6
Total revenue from contracts with customers \| Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	208.4	212.0
Total revenue from contracts with customers \| Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	44.8	60.5
Total revenue from contracts with customers \| All Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.1
Retail
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	211.0	226.9
Retail \| Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	89.5	101.0
Retail \| Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	119.9	124.3
Retail \| Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1.6	1.6
Retail \| Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	167.0	167.4
Retail \| Regulated Electric \| Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	65.0	67.4
Retail \| Regulated Electric \| Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	100.4	98.5
Retail \| Regulated Electric \| Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1.6	1.5
Retail \| Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	44.0	59.4
Retail \| Regulated Natural Gas \| Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	24.5	33.6
Retail \| Regulated Natural Gas \| Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	19.5	25.8
Retail \| Regulated Natural Gas \| Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0
Retail \| All Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.1
Retail \| All Other \| Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0
Retail \| All Other \| Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0
Retail \| All Other \| Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.1
Interchange [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	40.9	44.1
Interchange [Member] \| Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	40.9	44.1
Interchange [Member] \| Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0
Interchange [Member] \| All Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0
Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1.3	1.6
Other \| Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.5	0.5
Other \| Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.8	1.1
Other \| All Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0
Alternative revenue and other
Disaggregation of Revenue [Line Items]
Alternative revenue and other	3.8	3.6
Alternative revenue and other \| Regulated Electric
Disaggregation of Revenue [Line Items]
Alternative revenue and other	3.4	3.0
Alternative revenue and other \| Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Alternative revenue and other	0.4	0.6
Alternative revenue and other \| All Other
Disaggregation of Revenue [Line Items]
Alternative revenue and other	$ 0.0	$ 0.0

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+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of revenue that is not accounted for under Topic 606.

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of revenue recognized from goods sold, services rendered, insurance premiums, or other activities that constitute an earning process. Includes, but is not limited to, investment and interest income before deduction of interest expense when recognized as a component of revenue, and sales and trading gain (loss).

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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