REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Regulatory assets
Other current assets	$ 41.9	$ 24.9
Regulatory assets	3,393.1	3,249.8
Total regulatory assets	3,435.0	3,274.7
Plant retirement related items
Regulatory assets
Total regulatory assets	824.6	646.2
Plant retirement related items | Coal Combustion Residuals Rule
Regulatory assets
Total regulatory assets	116.0
Pension and OPEB costs
Regulatory assets
Total regulatory assets	725.1	731.7
Environmental remediation costs
Regulatory assets
Total regulatory assets	575.2	596.8
Income tax related items
Regulatory assets
Total regulatory assets	442.8	449.9
Asset retirement obligations
Regulatory assets
Total regulatory assets	167.1	162.0
Uncollectible expense
Regulatory assets
Total regulatory assets	123.9	127.7
System support resource
Regulatory assets
Total regulatory assets	107.9	113.2
Decoupling
Regulatory assets
Total regulatory assets	102.1	27.3
Securitization
Regulatory assets
Total regulatory assets	82.3	85.9
Derivatives
Regulatory assets
Total regulatory assets	62.7	130.3
Bluewater
Regulatory assets
Total regulatory assets	53.5	45.3
Energy efficiency programs
Regulatory assets
Total regulatory assets	30.1	33.9
Other, net
Regulatory assets
Total regulatory assets	$ 137.7	$ 124.5