Revenue from Contracts with Customers (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 420.4	$ 477.4
Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	166.9	191.2
Commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	91.3	95.4
Industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	41.5	52.3
Oilfield
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	38.2	49.4
Public authorities and street light
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	34.7	40.1
System sales revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	372.6	428.4
Provision for rate refund
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	(0.6)	(0.1)
Integrated market
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	7.2	6.7
Transmission
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	34.2	36.1
Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 7.0	$ 6.3