Regulatory Assets and Liabilities - Regulatory Assets (Details) - USD ($) $ in Millions	1 Months Ended		12 Months Ended
	Sep. 30, 2021	Dec. 31, 2019	Dec. 31, 2022	Dec. 31, 2021
Regulatory assets
Other current assets			$ 42.3	$ 102.3
Regulatory assets			3,264.6	3,264.8
Total regulatory assets			3,306.9	3,367.1
Allowance for return on equity capitalized for regulatory purposes			27.3	30.9
Regulatory assets not earning a return			237.9
Regulatory assets earning a return based on short-term interest rates			35.3
Regulatory assets earning a return based on long-term interest rates			123.5
Estimated future cash expenditures for environmental remediation			499.6	532.6
Pension and OPEB costs
Regulatory assets
Total regulatory assets			714.3	802.3
Plant retirement related items
Regulatory assets
Total regulatory assets			688.6	722.3
Environmental remediation costs
Regulatory assets
Total regulatory assets			610.7	630.9
Cash expenditures for environmental remediation costs			111.1
Estimated future cash expenditures for environmental remediation			499.6
Income tax related items
Regulatory assets
Total regulatory assets			461.9	458.8
Asset retirement obligations (AROs)
Regulatory assets
Total regulatory assets			169.7	194.2
Derivatives
Regulatory assets
Total regulatory assets			133.8	33.1
System support resource (SSR)
Regulatory assets
Total regulatory assets			123.5	129.5
Recovery period of regulatory asset		15 years
Securitization
Regulatory assets
Total regulatory assets			92.4	100.7
Uncollectible expense
Regulatory assets
Total regulatory assets			69.3	42.6
MERC extraordinary natural gas costs
Regulatory assets
Total regulatory assets			35.1	59.7
Recovery period of regulatory asset	27 months
Energy efficiency programs
Regulatory assets
Total regulatory assets			33.9	22.0
Energy costs recoverable through rate adjustments
Regulatory assets
Total regulatory assets			26.9	85.4
Other, net
Regulatory assets
Total regulatory assets			$ 146.8	$ 85.6