Revenue from Contracts with Customers (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenue from Contract with Customer, Excluding Assessed Tax	$ 2,211.7	$ 0.0	$ 0.0
Provision for Rate Refund [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	(6.0)
Integrated Market [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	48.7
Transmission [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	147.4
Other Contracts with Customers [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	27.1
Residential [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	877.8
Commercial [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	578.0
Industrial [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	191.1
Oilfield [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	150.2
Public Authority [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	197.4
Total Retail Customer [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 1,994.5