Revenues (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	$ 243.7	$ 256.0	$ 748.8	$ 760.9
Regulated Electric | Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	229.0	240.3	651.9	666.2
Regulated Natural Gas | Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	14.6	15.4	96.6	93.8
All Other | Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	0.1	0.3	0.3	0.9
Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	240.2	252.6	738.1	750.4
Total revenue from contracts with customers | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	226.1	237.5	643.0	657.5
Total revenue from contracts with customers | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	14.0	14.8	94.8	92.0
Total revenue from contracts with customers | All Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.1	0.3	0.3	0.9
Retail
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	194.2	211.3	601.9	628.2
Retail | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	70.5	74.9	236.4	242.7
Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	122.7	134.8	360.7	380.0
Retail | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1.0	1.6	4.8	5.5
Retail | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	181.1	197.1	509.8	538.3
Retail | Regulated Electric | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	63.6	67.9	185.1	194.5
Retail | Regulated Electric | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	116.6	127.9	320.2	339.1
Retail | Regulated Electric | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.9	1.3	4.5	4.7
Retail | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	13.0	13.9	91.8	89.0
Retail | Regulated Natural Gas | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	6.9	7.0	51.3	48.2
Retail | Regulated Natural Gas | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	6.1	6.9	40.5	40.8
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	0.3	0.3	0.9
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 | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0	0.0	0.1
Retail | All Other | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.1	0.3	0.3	0.8
Interchange [Member]
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	44.7	40.0	132.3	116.0
Interchange [Member] | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	44.7	40.0	132.3	116.0
Interchange [Member] | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.0	0.0	0.0	0.0
Interchange [Member] | 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	1.3	1.3	3.9	6.2
Other | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	0.3	0.4	0.9	3.2
Other | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1.0	0.9	3.0	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	3.5	3.4	10.7	10.5
Alternative revenue and other | Regulated Electric
Disaggregation of Revenue [Line Items]
Alternative revenue and other	2.9	2.8	8.9	8.7
Alternative revenue and other | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Alternative revenue and other	0.6	0.6	1.8	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