Revenues (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 966.6	$ 1,007.6
Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	785.3	841.5
Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	313.2	328.3
Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	466.1	506.0
Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	6.0	7.2
Interchange
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	176.6	157.9
Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	4.7	8.2
Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	14.3	13.9
Regulated Electric | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	832.2	867.1
Regulated Electric | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	654.5	705.6
Regulated Electric | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	240.2	255.2
Regulated Electric | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	408.7	444.3
Regulated Electric | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	5.6	6.1
Regulated Electric | Interchange
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	176.6	157.9
Regulated Electric | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	1.1	3.6
Regulated Electric | Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	11.9	11.5
Regulated Natural Gas | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	134.0	139.2
Regulated Natural Gas | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	130.4	134.6
Regulated Natural Gas | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	73.0	73.0
Regulated Natural Gas | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	57.4	61.6
Regulated Natural Gas | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.0	0.0
Regulated Natural Gas | Interchange
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.0	0.0
Regulated Natural Gas | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	3.6	4.6
Regulated Natural Gas | Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	2.4	2.4
All Other | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.4	1.3
All Other | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.4	1.3
All Other | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.0	0.1
All Other | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.0	0.1
All Other | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.4	1.1
All Other | Interchange
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.0	0.0
All Other | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.0	0.0
All Other | Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	0.0	0.0
Operating Segments
Disaggregation of Revenue [Line Items]
Operating revenues	980.9	1,021.5
Operating Segments | Regulated Electric
Disaggregation of Revenue [Line Items]
Operating revenues	844.1	878.6
Operating Segments | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Operating revenues	136.4	141.6
Operating Segments | All Other
Disaggregation of Revenue [Line Items]
Operating revenues	$ 0.4	$ 1.3