Regulatory Assets and Liabilities - Regulatory Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Regulatory assets
Current assets	$ 20.9	$ 50.7
Regulatory assets	3,506.7	3,805.1
Total regulatory assets	3,527.6	3,855.8
Allowance for return on equity capitalized for regulatory purposes	24.3	18.2
Regulatory assets not earning a return	175.1
Regulatory assets earning a return based on short-term interest rates	29.1
Regulatory assets earning a return based on long-term interest rates	151.5
Estimated future cash expenditures for environmental remediation	589.2	616.4
Pension and OPEB costs
Regulatory assets
Total regulatory assets	1,066.6	1,193.5
Plant retirements
Regulatory assets
Total regulatory assets	856.4	832.3
Environmental remediation costs
Regulatory assets
Total regulatory assets	685.5	687.1
Cash expenditures for environmental remediation costs	96.3
Estimated future cash expenditures for environmental remediation	589.2
Income tax related items
Regulatory assets
Total regulatory assets	457.8	369.1
System support resource (SSR)
Regulatory assets
Total regulatory assets	151.5	316.7
Asset retirement obligations (AROs)
Regulatory assets
Total regulatory assets	137.5	185.4
Uncollectible expense
Regulatory assets
Total regulatory assets	52.2	38.7
Derivatives
Regulatory assets
Total regulatory assets	33.8	17.8
We Power generation
Regulatory assets
Total regulatory assets	25.8	43.0
Electric transmission costs
Regulatory assets
Total regulatory assets	0.3	58.1
Refunds due to customers offset against electric transmission regulatory asset		37.2
Other, net
Regulatory assets
Total regulatory assets	$ 60.2	$ 114.1