Revenues from Contracts with Customers (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]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 501.1	$ 547.7	$ 978.5	$ 1,025.6
Residential [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	182.3	217.3	373.5	413.5
Commercial [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	115.4	135.1	210.8	235.9
Industrial [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	52.9	57.0	105.2	107.4
Oilfield [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	48.9	45.7	98.3	87.8
Public Authority [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	44.4	51.1	84.5	92.8
System sales revenues
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	443.9	506.2	872.3	937.4
Provision for Rate Refund [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	(0.5)	(16.5)	(0.6)	(19.7)
Integrated Market [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	10.3	13.1	17.0	21.8
Transmission [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	39.8	40.2	75.9	76.0
Other Contracts with Customers [Member]
Disaggregation of Revenue [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 7.6	$ 4.7	$ 13.9	$ 10.1