Revenues (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Operating revenues	$ 454.1	$ 447.2
Regulated Electric [Member] | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	446.7	441.9
Regulated Electric [Member] | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	303.5	285.5
Regulated Electric [Member] | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	88.1	80.0
Regulated Electric [Member] | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	205.8	195.8
Regulated Electric [Member] | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	9.6	9.7
Regulated Electric [Member] | Wholesale
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	84.8	93.2
Regulated Electric [Member] | Transmission
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	57.4	55.7
Regulated Electric [Member] | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	1.0	7.5
Regulated Electric [Member] | Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Operating revenues	7.4	5.3
Total revenues | Regulated Electric [Member]
Disaggregation of Revenue [Line Items]
Operating revenues	$ 454.1	$ 447.2