Revenue from Contracts with Customers (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 684.5	$ 0.0	$ 1,710.1	$ 0.0
Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	281.0		694.6
Commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	174.7		444.9
Industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	56.2		145.5
Oilfield
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	39.5		110.9
Public authorities and street light
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	58.7		151.5
System sales revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	610.1		1,547.4
Provision for rate refund
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	13.5		(6.2)
Integrated market
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	16.9		38.7
Transmission
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	33.2		109.2
Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 10.8		$ 21.0