Regulatory Assets and Liabilities - Regulatory Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Jan. 01, 2020
Regulatory assets
Other current assets	$ 39.0	$ 24.9
Regulatory assets	3,339.7	3,249.8
Total regulatory assets	3,378.7	3,274.7
Allowance for return on equity capitalized for regulatory purposes	26.7	26.7
Regulatory assets not earning a return	281.3
Regulatory assets earning a return based on short-term interest rates	2.3
Regulatory assets earning a return based on long-term interest rates	117.9
Regulatory assets earning a return based on return on equity rates	5.8
Estimated future cash expenditures for environmental remediation	445.8	463.7
Plant retirement related items
Regulatory assets
Total regulatory assets	810.5	646.2
Plant retirement related items | Coal Combustion Residual Rule
Regulatory assets
Total regulatory assets	121.3
Pension and OPEB costs
Regulatory assets
Total regulatory assets	684.9	731.7
Environmental remediation costs
Regulatory assets
Total regulatory assets	570.1	596.8
Cash expenditures for environmental remediation costs	124.3
Estimated future cash expenditures for environmental remediation	445.8
Income tax related items
Regulatory assets
Total regulatory assets	438.5	449.9
Asset retirement obligations (AROs)
Regulatory assets
Total regulatory assets	166.7	162.0
Uncollectible expense
Regulatory assets
Total regulatory assets	151.5	127.7
Decoupling
Regulatory assets
Total regulatory assets	110.0	27.3
System support resource (SSR)
Regulatory assets
Total regulatory assets	102.9	113.2
Recovery period of regulatory asset			15 years
Securitization
Regulatory assets
Total regulatory assets	76.5	85.9
Bluewater
Regulatory assets
Total regulatory assets	57.7	45.3
Derivatives
Regulatory assets
Total regulatory assets	38.2	130.3
Energy efficiency programs
Regulatory assets
Total regulatory assets	26.5	33.9
Finance and operating leases
Regulatory assets
Total regulatory assets	22.0	12.0
Other, net
Regulatory assets
Total regulatory assets	$ 122.7	$ 112.5