Revenues (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 228.9	$ 231.8	$ 505.1	$ 504.9
Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	225.3	228.3	497.9	497.8
Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	180.8	187.5	407.7	416.9
Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	64.9	69.6	165.9	167.8
Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	113.7	115.8	238.0	245.2
Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	2.2	2.1	3.8	3.9
Interchange [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	43.5	38.4	87.6	76.0
Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	1.0	2.4	2.6	4.9
Alternative revenue and other
Disaggregation of Revenue [Line Items]
Total operating revenues	3.6	3.5	7.2	7.1
Regulated Electric | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	204.9	206.6	416.9	420.0
Regulated Electric | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	161.3	166.9	328.7	341.1
Regulated Electric | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	54.1	58.7	121.5	126.5
Regulated Electric | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	105.1	106.4	203.6	211.2
Regulated Electric | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	2.1	1.8	3.6	3.4
Regulated Electric | Interchange [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	43.5	38.4	87.6	76.0
Regulated Electric | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.1	1.3	0.6	2.9
Regulated Electric | Alternative revenue and other
Disaggregation of Revenue [Line Items]
Total operating revenues	3.0	2.9	6.0	5.9
Regulated Natural Gas | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	20.3	21.4	80.8	77.3
Regulated Natural Gas | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	19.4	20.3	78.8	75.3
Regulated Natural Gas | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	10.8	10.9	44.4	41.3
Regulated Natural Gas | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	8.6	9.4	34.4	34.0
Regulated Natural Gas | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.0	0.0	0.0	0.0
Regulated Natural Gas | Interchange [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.0	0.0	0.0	0.0
Regulated Natural Gas | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.9	1.1	2.0	2.0
Regulated Natural Gas | Alternative revenue and other
Disaggregation of Revenue [Line Items]
Total operating revenues	0.6	0.6	1.2	1.2
All Other | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.1	0.3	0.2	0.5
All Other | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.1	0.3	0.2	0.5
All Other | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.0	0.0	0.0	0.0
All Other | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.0	0.0	0.0	0.0
All Other | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.1	0.3	0.2	0.5
All Other | Interchange [Member]
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.0	0.0	0.0	0.0
All Other | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.0	0.0	0.0	0.0
All Other | Alternative revenue and other
Disaggregation of Revenue [Line Items]
Total operating revenues	0.0	0.0	0.0	0.0
Total revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	228.9	231.8	505.1	504.9
Total revenues | Regulated Electric
Disaggregation of Revenue [Line Items]
Total operating revenues	207.9	209.5	422.9	425.9
Total revenues | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Total operating revenues	20.9	22.0	82.0	78.5
Total revenues | All Other
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 0.1	$ 0.3	$ 0.2	$ 0.5