Revenues (Details) - Regulated Electric - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	$ 537.3	$ 526.1	$ 1,339.0	$ 1,376.9
Total revenues
Disaggregation of Revenue [Line Items]
Total revenues	615.3	533.1	1,433.7	1,397.7
Retail
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	353.0	354.5	860.1	929.7
Retail | Residential
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	120.6	119.3	278.3	277.8
Retail | C&I
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	219.5	222.4	552.3	619.6
Retail | Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	12.9	12.8	29.5	32.3
Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	109.4	106.6	264.2	263.4
Transmission
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	73.5	64.4	212.1	181.8
Other
Disaggregation of Revenue [Line Items]
Revenue from contracts with customers	1.4	0.6	2.6	2.0
Alternative and Other
Disaggregation of Revenue [Line Items]
Alternative revenue and other	$ 78.0	$ 7.0	$ 94.7	$ 20.8