Regulatory Assets and Liabilities - Regulatory Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Regulatory assets
Other current assets	$ 102.3	$ 20.0
Regulatory assets	3,264.8	3,524.1
Total regulatory assets	3,367.1	3,544.1
Allowance for return on equity capitalized for regulatory purposes	30.9	34.2
Regulatory assets not earning a return	337.7
Regulatory assets earning a return based on short-term interest rates	14.3
Regulatory assets earning a return based on long-term interest rates	129.5
Estimated future cash expenditures for environmental remediation	532.6	532.9
Pension and OPEB costs
Regulatory assets
Total regulatory assets	802.3	1,101.6
Plant retirement related items
Regulatory assets
Total regulatory assets	722.3	740.8
Environmental remediation costs
Regulatory assets
Total regulatory assets	630.9	638.2
Cash expenditures for environmental remediation costs	98.3
Estimated future cash expenditures for environmental remediation	532.6
Income tax related items
Regulatory assets
Total regulatory assets	458.8	454.6
Asset retirement obligations (AROs)
Regulatory assets
Total regulatory assets	194.2	181.3
System support resource (SSR)
Regulatory assets
Total regulatory assets	$ 129.5	135.6
Recovery period of regulatory asset	15 years
Securitization
Regulatory assets
Total regulatory assets	$ 100.7	105.2
Energy costs recoverable through rate adjustments
Regulatory assets
Total regulatory assets	85.4	1.1
MERC extraordinary natural gas costs
Regulatory assets
Total regulatory assets	$ 59.7	0.0
Recovery period of regulatory asset	27 months
Uncollectible expense
Regulatory assets
Total regulatory assets	$ 42.6	82.0
Derivatives
Regulatory assets
Total regulatory assets	33.1	26.5
Energy efficiency programs
Regulatory assets
Total regulatory assets	22.0	7.3
Other, net
Regulatory assets
Total regulatory assets	$ 85.6	$ 69.9