Revenues (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 865.4	$ 866.2	$ 1,879.9	$ 2,048.6
Retail
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	772.2	795.7	1,681.4	1,864.2
Retail | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	364.3	344.4	822.3	903.4
Retail | Commercial and Industrial Customers [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	396.2	439.5	835.2	936.5
Retail | Other Customers [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	11.7	11.8	23.9	24.3
Wholesale Distribution [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	41.5	29.1	88.8	86.4
Transmission Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	13.8	11.6	27.2	25.0
Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	37.9	29.8	82.5	73.0
Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	45.7	43.8	87.9	84.4
Regulated Electric | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	676.8	654.9	1,354.0	1,362.7
Regulated Electric | Retail
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	607.0	607.7	1,208.7	1,233.2
Regulated Electric | Retail | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	243.8	211.5	481.3	456.6
Regulated Electric | Retail | Commercial and Industrial Customers [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	351.5	384.4	703.5	752.3
Regulated Electric | Retail | Other Customers [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	11.7	11.8	23.9	24.3
Regulated Electric | Wholesale Distribution [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	41.5	29.1	88.8	86.4
Regulated Electric | Transmission Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	13.8	11.6	27.2	25.0
Regulated Electric | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	14.5	6.5	29.3	18.1
Regulated Electric | Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	38.6	37.8	73.2	71.5
Regulated Natural Gas | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	180.2	202.8	505.2	666.1
Regulated Natural Gas | Retail
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	156.8	179.5	452.0	611.2
Regulated Natural Gas | Retail | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	117.7	130.1	335.1	441.5
Regulated Natural Gas | Retail | Commercial and Industrial Customers [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	39.1	49.4	116.9	169.7
Regulated Natural Gas | Retail | Other Customers [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Regulated Natural Gas | Wholesale Distribution [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Regulated Natural Gas | Transmission Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
Regulated Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	23.4	23.3	53.2	54.9
Regulated Natural Gas | Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	6.0	4.8	12.5	10.6
All Other | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	8.4	8.5	20.7	19.8
All Other | Retail
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	8.4	8.5	20.7	19.8
All Other | Retail | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	2.8	2.8	5.9	5.3
All Other | Retail | Commercial and Industrial Customers [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	5.6	5.7	14.8	14.5
All Other | Retail | Other Customers [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
All Other | Wholesale Distribution [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
All Other | Transmission Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
All Other | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0.0	0.0	0.0	0.0
All Other | Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	1.1	1.2	2.2	2.3
Total revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	911.1	910.0	1,967.8	2,133.0
Total revenues | Regulated Electric
Disaggregation of Revenue [Line Items]
Total operating revenues	715.4	692.7	1,427.2	1,434.2
Total revenues | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Total operating revenues	186.2	207.6	517.7	676.7
Total revenues | All Other
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 9.5	$ 9.7	$ 22.9	$ 22.1