Revenue from Contracts with Customers (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue from Contract with Customer, Excluding Assessed Tax	$ 2,069.8	$ 2,175.5	$ 2,211.7
Provision for Rate Refund [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	3.8	(0.9)	(6.0)
Integrated Market [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	49.6	38.4	48.7
Transmission [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	143.3	148.0	147.4
Other Contracts with Customers [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	30.7	29.1	27.1
Residential [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	842.7	865.8	877.8
Commercial [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	465.6	486.6	500.0
Industrial [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	192.6	217.8	228.9
Oilfield [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	169.2	200.4	190.4
Public Authority [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	172.3	190.3	197.4
Total Retail Customer [Member]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 1,842.4	$ 1,960.9	$ 1,994.5