Regulatory Assets and Liabilities - Regulatory Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Regulatory assets
Other current assets	$ 20.0	$ 20.9
Regulatory assets	3,524.1	3,506.7
Total regulatory assets	3,544.1	3,527.6
Allowance for return on equity capitalized for regulatory purposes	34.2	24.3
Regulatory assets not earning a return	238.8
Regulatory assets earning a return based on short-term interest rates	9.7
Regulatory assets earning a return based on long-term interest rates	135.6
Estimated future cash expenditures for environmental remediation	532.9	589.2
Pension and OPEB costs
Regulatory assets
Total regulatory assets	1,101.6	1,066.6
Plant retirements
Regulatory assets
Total regulatory assets	740.8	756.4
Environmental remediation costs
Regulatory assets
Total regulatory assets	638.2	685.5
Cash expenditures for environmental remediation costs	105.3
Estimated future cash expenditures for environmental remediation	532.9
Income tax related items
Regulatory assets
Total regulatory assets	454.6	457.8
Asset retirement obligations (AROs)
Regulatory assets
Total regulatory assets	181.3	137.5
System support resource (SSR)
Regulatory assets
Total regulatory assets	$ 135.6	151.5
Recovery period of regulatory asset	15 years
Securitization
Regulatory assets
Total regulatory assets	$ 105.2	100.0
Uncollectible expense
Regulatory assets
Total regulatory assets	82.0	52.2
Derivatives
Regulatory assets
Total regulatory assets	26.5	33.8
We Power generation
Regulatory assets
Total regulatory assets	7.6	25.8
Other, net
Regulatory assets
Total regulatory assets	$ 70.7	$ 60.5