Revenue from Contracts with Customers (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]
Revenues from contracts with customers	$ 739.2	$ 684.5	$ 1,717.7	$ 1,710.1
Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	318.9	281.0	692.4	694.6
Commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	172.9	150.8	383.7	386.6
Industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	66.8	67.2	172.0	174.6
Oilfield
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	58.6	52.4	156.9	140.1
Public authorities and street light
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	65.7	58.7	150.2	151.5
System sales revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	682.9	610.1	1,555.2	1,547.4
Provision for rate refund
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	(2.3)	13.5	(2.9)	(6.2)
Integrated market
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	12.8	16.9	29.8	38.7
Transmission
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	36.7	33.2	112.6	109.2
Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 9.1	$ 10.8	$ 23.0	$ 21.0