Revenue from Contracts with Customers (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 477.4	$ 477.9
Residential
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	191.2	196.3
Commercial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	115.1	117.8
Industrial
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	43.1	41.7
Oilfield
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	38.9	33.7
Public authorities and street light
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	40.1	41.7
System sales revenues
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	428.4	431.2
Provision for rate refund
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	(0.1)	(3.2)
Integrated market
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	6.7	8.6
Transmission
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	36.1	35.8
Other
Disaggregation of Revenue [Line Items]
Revenues from contracts with customers	$ 6.3	$ 5.5