Revenues (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Total revenues	$ 1,056.7	$ 1,223.0
Regulated Electric
Disaggregation of Revenue [Line Items]
Total revenues	711.8	741.5
Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Total revenues	331.5	469.1
All Other
Disaggregation of Revenue [Line Items]
Total revenues	13.4	12.4
Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	1,014.5	1,182.4
Total revenue from contracts with customers | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	677.2	707.8
Total revenue from contracts with customers | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	325.0	463.3
Total revenue from contracts with customers | All Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	12.3	11.3
Retail
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	909.2	1,068.5
Retail | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	458.0	559.0
Retail | C&I
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	439.0	497.0
Retail | Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	12.2	12.5
Retail | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	601.7	625.5
Retail | Regulated Electric | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	237.5	245.1
Retail | Regulated Electric | C&I
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	352.0	367.9
Retail | Regulated Electric | Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	12.2	12.5
Retail | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	295.2	431.7
Retail | Regulated Natural Gas | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	217.4	311.4
Retail | Regulated Natural Gas | C&I
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	77.8	120.3
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	12.3	11.3
Retail | All Other | Residential
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	3.1	2.5
Retail | All Other | C&I
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	9.2	8.8
Retail | All Other | Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0	0.0
Wholesale
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	47.3	57.3
Wholesale | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	47.3	57.3
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	13.4	13.4
Transmission | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	13.4	13.4
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
Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	44.6	43.2
Other | Regulated Electric
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	14.8	11.6
Other | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	29.8	31.6
Other | All Other
Disaggregation of Revenue [Line Items]
Revenue from contract with customer	0.0	0.0
Alternative and other
Disaggregation of Revenue [Line Items]
Alternative revenue and other	42.2	40.6
Alternative and other | Regulated Electric
Disaggregation of Revenue [Line Items]
Alternative revenue and other	34.6	33.7
Alternative and other | Regulated Natural Gas
Disaggregation of Revenue [Line Items]
Alternative revenue and other	6.5	5.8
Alternative and other | All Other
Disaggregation of Revenue [Line Items]
Alternative revenue and other	$ 1.1	$ 1.1