Revenues Revenues (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	$ 4,839.4	$ 4,916.9
Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	3,932.9	3,975.9
Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	1,613.3	1,644.4
Retail | Commercial and Industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	2,282.9	2,291.6
Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	36.7	39.9
Wholesale
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	210.1	189.2
Transmission
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	216.0	238.1
Interchange
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	458.7	473.7
Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	21.7	40.0
Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	272.4	205.0
Regulated Electric | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	4,264.1	4,326.3
Regulated Electric | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	3,367.4	3,397.0
Regulated Electric | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	1,280.2	1,308.4
Regulated Electric | Retail | Commercial and Industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	2,054.3	2,052.1
Regulated Electric | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	32.9	36.5
Regulated Electric | Wholesale
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	210.1	189.2
Regulated Electric | Transmission
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	216.0	238.1
Regulated Electric | Interchange
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	458.7	473.7
Regulated Electric | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	11.9	28.3
Regulated Electric | Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	242.5	181.7
Regulated Natural Gas | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	541.4	559.8
Regulated Natural Gas | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	531.6	548.1
Regulated Natural Gas | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	303.0	308.8
Regulated Natural Gas | Retail | Commercial and Industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	228.6	239.3
Regulated Natural Gas | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.0	0.0
Regulated Natural Gas | Wholesale
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.0	0.0
Regulated Natural Gas | Transmission
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.0	0.0
Regulated Natural Gas | Interchange
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.0	0.0
Regulated Natural Gas | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	9.8	11.7
Regulated Natural Gas | Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	29.9	23.3
All Other | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	33.9	30.8
All Other | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	33.9	30.8
All Other | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	30.1	27.2
All Other | Retail | Commercial and Industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.0	0.2
All Other | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	3.8	3.4
All Other | Wholesale
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.0	0.0
All Other | Transmission
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.0	0.0
All Other | Interchange
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.0	0.0
All Other | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.0	0.0
All Other | Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Revenue Not from Contract with Customer	0.0	0.0
Operating Segments
Disaggregation of Revenue [Line Items]
Total operating revenues	5,111.8	5,121.9
Operating Segments | Regulated Electric
Disaggregation of Revenue [Line Items]
Total operating revenues	4,506.6	4,508.0
Operating Segments | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Total operating revenues	571.3	583.1
Operating Segments | All Other
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 33.9	$ 30.8