Revenues (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	$ 1,039.4	$ 1,001.5	$ 2,919.3	$ 3,050.1
Retail
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	922.6	915.1	2,604.0	2,779.3
Retail | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	442.7	412.5	1,265.0	1,315.9
Retail | Commercial and Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	468.6	490.8	1,303.8	1,427.3
Retail | Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	11.3	11.8	35.2	36.1
Wholesale Distribution [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	61.2	40.3	150.0	126.7
Transmission Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	17.8	16.4	45.0	41.4
Other
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	37.8	29.7	120.3	102.7
Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Alternative revenue and other	41.7	42.8	129.6	127.2
Regulated Electric | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	879.3	844.5	2,233.3	2,207.2
Regulated Electric | Retail
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	786.6	779.5	1,995.3	2,012.7
Regulated Electric | Retail | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	344.1	315.3	825.4	771.9
Regulated Electric | Retail | Commercial and Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	431.2	452.4	1,134.7	1,204.7
Regulated Electric | Retail | Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	11.3	11.8	35.2	36.1
Regulated Electric | Wholesale Distribution [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	61.2	40.3	150.0	126.7
Regulated Electric | Transmission Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	17.8	16.4	45.0	41.4
Regulated Electric | Other
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	13.7	8.3	43.0	26.4
Regulated Electric | Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Alternative revenue and other	36.6	36.2	109.8	107.6
Regulated Natural Gas | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	152.4	148.4	657.6	814.5
Regulated Natural Gas | Retail
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	128.3	127.0	580.3	738.2
Regulated Natural Gas | Retail | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	95.5	94.1	430.6	535.6
Regulated Natural Gas | Retail | Commercial and Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	32.8	32.9	149.7	202.6
Regulated Natural Gas | Retail | Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0.0	0.0	0.0	0.0
Regulated Natural Gas | Wholesale Distribution [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0.0	0.0	0.0	0.0
Regulated Natural Gas | Transmission Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0.0	0.0	0.0	0.0
Regulated Natural Gas | Other
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	24.1	21.4	77.3	76.3
Regulated Natural Gas | Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Alternative revenue and other	4.0	5.5	16.5	16.2
All Other | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	7.7	8.6	28.4	28.4
All Other | Retail
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	7.7	8.6	28.4	28.4
All Other | Retail | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	3.1	3.1	9.0	8.4
All Other | Retail | Commercial and Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	4.6	5.5	19.4	20.0
All Other | Retail | Other [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0.0	0.0	0.0	0.0
All Other | Wholesale Distribution [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0.0	0.0	0.0	0.0
All Other | Transmission Services [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0.0	0.0	0.0	0.0
All Other | Other
Disaggregation of Revenue [Line Items]
Revenue from Contracts with Customers	0.0	0.0	0.0	0.0
All Other | Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Alternative revenue and other	1.1	1.1	3.3	3.4
Total revenues
Disaggregation of Revenue [Line Items]
Total operating revenues	1,081.1	1,044.3	3,048.9	3,177.3
Total revenues | Regulated Electric
Disaggregation of Revenue [Line Items]
Total operating revenues	915.9	880.7	2,343.1	2,314.8
Total revenues | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Total operating revenues	156.4	153.9	674.1	830.7
Total revenues | All Other
Disaggregation of Revenue [Line Items]
Total operating revenues	$ 8.8	$ 9.7	$ 31.7	$ 31.8