Revenues (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	$ 250.5	$ 243.7	$ 726.1	$ 748.8
Regulated Electric | Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	235.9	229.0	647.1	651.9
Regulated Natural Gas | Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	14.5	14.6	78.7	96.6
All Other | Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	0.1	0.1	0.3	0.3
Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	246.6	240.2	714.9	738.1
Total revenue from contracts with customers | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	232.5	226.1	637.3	643.0
Total revenue from contracts with customers | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	14.0	14.0	77.3	94.8
Total revenue from contracts with customers | All Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.1	0.1	0.3	0.3
Retail
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	204.0	194.2	587.4	601.9
Retail | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	77.9	70.5	237.6	236.4
Retail | C&I
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	125.0	122.7	344.9	360.7
Retail | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1.1	1.0	4.9	4.8
Retail | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	190.8	181.1	512.6	509.8
Retail | Regulated Electric | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	70.5	63.6	195.1	185.1
Retail | Regulated Electric | C&I
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	119.3	116.6	312.9	320.2
Retail | Regulated Electric | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1.0	0.9	4.6	4.5
Retail | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	13.1	13.0	74.5	91.8
Retail | Regulated Natural Gas | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	7.4	6.9	42.5	51.3
Retail | Regulated Natural Gas | C&I
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	5.7	6.1	32.0	40.5
Retail | Regulated Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0	0.0	0.0
Retail | All Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.1	0.1	0.3	0.3
Retail | All Other | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0	0.0	0.0
Retail | All Other | C&I
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0	0.0	0.0
Retail | All Other | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.1	0.1	0.3	0.3
Interchange
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	41.0	44.7	123.4	132.3
Interchange | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	41.0	44.7	123.4	132.3
Interchange | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0	0.0	0.0
Interchange | All Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0	0.0	0.0
Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1.6	1.3	4.1	3.9
Other | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.7	0.3	1.3	0.9
Other | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.9	1.0	2.8	3.0
Other | All Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0	0.0	0.0
Alternative revenue and other
Disaggregation of Revenue [Line Items]
Alternative revenue and other	3.9	3.5	11.2	10.7
Alternative revenue and other | Regulated Electric
Disaggregation of Revenue [Line Items]
Alternative revenue and other	3.4	2.9	9.8	8.9
Alternative revenue and other | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Alternative revenue and other	0.5	0.6	1.4	1.8
Alternative revenue and other | All Other
Disaggregation of Revenue [Line Items]
Alternative revenue and other	$ 0.0	$ 0.0	$ 0.0	$ 0.0