Revenues (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Disaggregation of Revenue [Line Items]
Operating revenues	$ 410.5	$ 481.3	$ 864.6	$ 928.6
Regulated Electric [Member] | Total revenue from contracts with customers
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	404.1	474.7	850.8	916.6
Regulated Electric [Member] | Retail
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	271.7	297.2	575.2	582.7
Regulated Electric [Member] | Retail | Residential
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	70.4	85.1	158.5	165.2
Regulated Electric [Member] | Retail | Commercial and industrial (C&I)
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	191.4	200.8	397.2	396.5
Regulated Electric [Member] | Retail | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	9.9	11.3	19.5	21.0
Regulated Electric [Member] | Wholesale
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	72.0	115.6	156.8	208.9
Regulated Electric [Member] | Transmission
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	60.0	59.0	117.4	114.6
Regulated Electric [Member] | Other
Disaggregation of Revenue [Line Items]
Total revenue from contracts with customers	0.4	2.9	1.4	10.4
Regulated Electric [Member] | Alternative and Other [Member]
Disaggregation of Revenue [Line Items]
Operating revenues	6.4	6.6	13.8	12.0
Total revenues | Regulated Electric [Member]
Disaggregation of Revenue [Line Items]
Operating revenues	$ 410.5	$ 481.3	$ 864.6	$ 928.6