Revenues (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation of Revenue [Line Items]
Revenues	$ 464.6	$ 540.1	$ 481.3	$ 447.2	$ 426.5	$ 551.6	$ 479.8	$ 460.1	$ 1,933.2	$ 1,918.0	$ 1,851.0
Regulated Electric [Member] | Retail
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									1,236.7
Regulated Electric [Member] | Retail | Residential
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									363.7
Regulated Electric [Member] | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									828.3
Regulated Electric [Member] | Retail | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									44.7
Regulated Electric [Member] | Wholesale
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									426.0
Regulated Electric [Member] | Transmission
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									231.1
Regulated Electric [Member] | Other
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									12.8
Regulated Electric [Member] | Product [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Including Assessed Tax									1,906.6
Regulated Electric [Member] | Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Revenues									26.6
Operating Segments [Member] | Regulated Electric [Member]
Disaggregation of Revenue [Line Items]
Revenues									$ 1,933.2