Revenues (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Total revenues	$ 1,044.3	$ 1,060.7	$ 3,177.3	$ 3,045.8
Regulated Electric
Disaggregation of Revenue [Line Items]
Total revenues	880.7	894.8	2,314.8	2,309.3
Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Total revenues	153.9	157.2	830.7	707.8
All Other
Disaggregation of Revenue [Line Items]
Total revenues	9.7	8.7	31.8	28.7
Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,001.5	993.6	3,050.1	2,855.7
Total revenue from contracts with customers | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	844.5	854.1	2,207.2	2,198.0
Total revenue from contracts with customers | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	148.4	131.9	814.5	632.3
Total revenue from contracts with customers | All Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	8.6	7.6	28.4	25.4
Retail
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	915.1	907.1	2,779.3	2,582.5
Retail | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	412.5	402.8	1,315.9	1,190.2
Retail | C&I
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	490.8	492.5	1,427.3	1,356.8
Retail | Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	11.8	11.8	36.1	35.5
Retail | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	779.5	786.0	2,012.7	1,986.9
Retail | Regulated Electric | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	315.3	315.7	771.9	766.1
Retail | Regulated Electric | C&I
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	452.4	458.5	1,204.7	1,185.4
Retail | Regulated Electric | Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	11.8	11.8	36.1	35.4
Retail | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	127.0	113.5	738.2	570.2
Retail | Regulated Natural Gas | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	94.1	84.4	535.6	416.2
Retail | Regulated Natural Gas | C&I
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	32.9	29.1	202.6	154.0
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	8.6	7.6	28.4	25.4
Retail | All Other | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	3.1	2.7	8.4	7.9
Retail | All Other | C&I
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	5.5	4.9	20.0	17.4
Retail | All Other | Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0	0.0	0.0	0.1
Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	40.3	41.2	126.7	125.3
Wholesale | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	40.3	41.2	126.7	125.3
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	16.4	16.3	41.4	41.7
Transmission | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	16.4	16.3	41.4	41.7
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
Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	29.7	29.0	102.7	106.2
Other | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	8.3	10.6	26.4	44.1
Other | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	21.4	18.4	76.3	62.1
Other | All Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0	0.0	0.0	0.0
Alternative and other
Disaggregation of Revenue [Line Items]
Alternative revenue and other	42.8	67.1	127.2	190.1
Alternative and other | Regulated Electric
Disaggregation of Revenue [Line Items]
Alternative revenue and other	36.2	40.7	107.6	111.3
Alternative and other | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Alternative revenue and other	5.5	25.3	16.2	75.5
Alternative and other | All Other
Disaggregation of Revenue [Line Items]
Alternative revenue and other	$ 1.1	$ 1.1	$ 3.4	$ 3.3