Revenues (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Operating revenues	$ 260.6	$ 256.0	$ 231.8	$ 273.1	$ 263.0	$ 247.5	$ 230.1	$ 264.9	$ 1,021.5	$ 1,005.5	$ 957.2
Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									1,007.6
Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									841.5
Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									328.3
Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									506.0
Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									7.2
Interchange
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									157.9
Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									8.2
Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Operating revenues									13.9
Regulated Electric | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									867.1
Regulated Electric | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									705.6
Regulated Electric | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									255.2
Regulated Electric | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									444.3
Regulated Electric | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									6.1
Regulated Electric | Interchange
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									157.9
Regulated Electric | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									3.6
Regulated Electric | Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Operating revenues									11.5
Regulated Natural Gas | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									139.2
Regulated Natural Gas | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									134.6
Regulated Natural Gas | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									73.0
Regulated Natural Gas | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									61.6
Regulated Natural Gas | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									0.0
Regulated Natural Gas | Interchange
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									0.0
Regulated Natural Gas | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									4.6
Regulated Natural Gas | Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Operating revenues									2.4
All Other | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									1.3
All Other | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									1.3
All Other | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									0.1
All Other | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									0.1
All Other | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									1.1
All Other | Interchange
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									0.0
All Other | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers									0.0
All Other | Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Operating revenues									0.0
Operating Segments
Disaggregation of Revenue [Line Items]
Operating revenues									1,021.5
Operating Segments | Regulated Electric
Disaggregation of Revenue [Line Items]
Operating revenues									878.6
Operating Segments | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Operating revenues									141.6
Operating Segments | All Other
Disaggregation of Revenue [Line Items]
Operating revenues									$ 1.3