Revenues (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	$ 1,179.9	$ 1,185.0	$ 2,430.1	$ 2,535.5
Regulated Electric | Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	1,095.7	1,097.2	2,131.0	2,175.9
Regulated Natural Gas | Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	75.0	79.4	281.1	343.5
All Other | Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	9.2	8.4	18.0	16.1
Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,094.6	1,126.8	2,265.9	2,428.9
Total revenue from contracts with customers | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,018.1	1,043.7	1,983.7	2,078.6
Total revenue from contracts with customers | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	67.3	74.7	264.2	334.2
Total revenue from contracts with customers | All Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	9.2	8.4	18.0	16.1
Retail
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	879.4	896.5	1,836.6	1,965.3
Retail | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	375.7	336.1	796.7	819.5
Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	493.6	551.5	1,020.4	1,127.8
Retail | Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	10.1	8.9	19.5	18.0
Retail | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	804.9	815.2	1,557.7	1,617.9
Retail | Regulated Electric | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	329.5	288.1	629.6	615.0
Retail | Regulated Electric | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	466.9	519.5	911.6	987.0
Retail | Regulated Electric | Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	8.5	7.6	16.5	15.9
Retail | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	65.3	72.9	260.9	331.3
Retail | Regulated Natural Gas | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	38.6	40.9	152.1	190.5
Retail | Regulated Natural Gas | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	26.7	32.0	108.8	140.8
Retail | Regulated Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0	0.0	0.0	0.0
Retail | All Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	9.2	8.4	18.0	16.1
Retail | All Other | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	7.6	7.1	15.0	14.0
Retail | All Other | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0	0.0	0.0	0.0
Retail | All Other | Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1.6	1.3	3.0	2.1
Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	37.2	53.1	82.6	100.0
Wholesale | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	37.2	53.1	82.6	100.0
Wholesale | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0	0.0	0.0	0.0
Wholesale | All Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0	0.0	0.0	0.0
Transmission
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	63.5	55.3	119.4	115.7
Transmission | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	63.5	55.3	119.4	115.7
Transmission | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0	0.0	0.0	0.0
Transmission | All Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0	0.0	0.0	0.0
Interchange [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	106.1	116.2	215.7	236.3
Interchange [Member] | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	106.1	116.2	215.7	236.3
Interchange [Member] | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0	0.0	0.0	0.0
Interchange [Member] | All Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0	0.0	0.0	0.0
Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	8.4	5.7	11.6	11.6
Other | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	6.4	3.9	8.3	8.7
Other | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	2.0	1.8	3.3	2.9
Other | All Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0	0.0	0.0	0.0
Alternative revenue and other
Disaggregation of Revenue [Line Items]
Alternative revenue and other	85.3	58.2	164.2	106.6
Alternative revenue and other | Regulated Electric
Disaggregation of Revenue [Line Items]
Alternative revenue and other	77.6	53.5	147.3	97.3
Alternative revenue and other | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Alternative revenue and other	7.7	4.7	16.9	9.3
Alternative revenue and other | All Other
Disaggregation of Revenue [Line Items]
Alternative revenue and other	$ 0.0	$ 0.0	$ 0.0	$ 0.0