REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Regulatory assets
Other current assets	$ 24.2	$ 39.0
Regulatory assets	3,231.8	3,339.7
Total regulatory assets	3,256.0	3,378.7
Plant retirement related items
Regulatory assets
Total regulatory assets	800.6	810.5
Pension and OPEB costs
Regulatory assets
Total regulatory assets	637.5	684.9
Environmental remediation costs
Regulatory assets
Total regulatory assets	527.3	570.1
Income tax related items
Regulatory assets
Total regulatory assets	465.5	438.5
Asset retirement obligations
Regulatory assets
Total regulatory assets	177.0	166.7
Uncollectible expense
Regulatory assets
Total regulatory assets	122.1	151.5
Decoupling
Regulatory assets
Total regulatory assets	110.8	110.0
System support resource
Regulatory assets
Total regulatory assets	97.7	102.9
Securitization
Regulatory assets
Total regulatory assets	72.2	76.5
Bluewater
Regulatory assets
Total regulatory assets	49.6	57.7
Finance and operating leases
Regulatory assets
Total regulatory assets	27.8	22.0
Derivatives
Regulatory assets
Total regulatory assets	20.7	38.2
Energy efficiency programs
Regulatory assets
Total regulatory assets	19.1	26.5
Energy costs recoverable through rate adjustments
Regulatory assets
Total regulatory assets	15.9	8.3
Other, net
Regulatory assets
Total regulatory assets	$ 112.2	$ 114.4