Revenue from Contracts with Customers (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Revenue from Contract with Customer, Excluding Assessed Tax	$ 2,175.5	$ 2,211.7	$ 0.0
Provision for Rate Refund [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	(0.9)	(6.0)
Integrated Market [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	38.4	48.7
Transmission [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	148.0	147.4
Other Contracts with Customers [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	29.1	27.1
Residential [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	865.8	877.8
Commercial [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	486.6	500.0
Industrial [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	217.8	228.9
Oilfield [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	200.4	190.4
Public Authority [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	190.3	197.4
Total Retail Customer [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 1,960.9	$ 1,994.5