Revenues (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	$ 1,250.2	$ 1,350.5
Regulated Electric | Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	1,035.2	1,078.7
Regulated Natural Gas | Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	206.1	264.0
All Other | Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	8.9	7.8
Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,171.2	1,302.1
Total revenue from contracts with customers | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	965.5	1,034.9
Total revenue from contracts with customers | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	196.8	259.4
Total revenue from contracts with customers | All Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	8.9	7.8
Retail
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	957.2	1,068.9
Retail | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	421.0	483.4
Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	526.8	576.3
Retail | Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	9.4	9.2
Retail | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	752.7	802.7
Retail | Regulated Electric | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	300.1	326.9
Retail | Regulated Electric | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	444.7	467.5
Retail | Regulated Electric | Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	7.9	8.3
Retail | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	195.6	258.4
Retail | Regulated Natural Gas | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	113.5	149.6
Retail | Regulated Natural Gas | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	82.1	108.8
Retail | Regulated Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0	0.0
Retail | All Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	8.9	7.8
Retail | All Other | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	7.4	6.9
Retail | All Other | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0	0.0
Retail | All Other | Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1.5	0.9
Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	45.4	46.9
Wholesale | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	45.4	46.9
Wholesale | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0	0.0
Wholesale | All Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0	0.0
Transmission
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	55.9	60.4
Transmission | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	55.9	60.4
Transmission | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0	0.0
Transmission | All Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0	0.0
Interchange [Member]
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	109.6	120.1
Interchange [Member] | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	109.6	120.1
Interchange [Member] | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0	0.0
Interchange [Member] | All Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0	0.0
Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	3.1	5.8
Other | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1.9	4.8
Other | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1.2	1.0
Other | All Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0	0.0
Alternative revenue and other
Disaggregation of Revenue [Line Items]
Alternative revenue and other	79.0	48.4
Alternative revenue and other | Regulated Electric
Disaggregation of Revenue [Line Items]
Alternative revenue and other	69.7	43.8
Alternative revenue and other | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Alternative revenue and other	9.3	4.6
Alternative revenue and other | All Other
Disaggregation of Revenue [Line Items]
Alternative revenue and other	$ 0.0	$ 0.0