REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Millions	Sep. 30, 2024	Dec. 31, 2023
Regulatory assets
Other current assets	$ 59.1	$ 24.9
Regulatory assets	3,346.5	3,249.8
Total regulatory assets	3,405.6	3,274.7
Plant retirement related items
Regulatory assets
Total regulatory assets	814.3	646.2
Plant retirement related items | Coal Combustion Residuals Rule
Regulatory assets
Total regulatory assets	115.0
Pension and OPEB costs
Regulatory assets
Total regulatory assets	721.1	731.7
Environmental remediation costs
Regulatory assets
Total regulatory assets	565.8	596.8
Income tax related items
Regulatory assets
Total regulatory assets	438.9	449.9
Asset retirement obligations
Regulatory assets
Total regulatory assets	169.3	162.0
Uncollectible expense
Regulatory assets
Total regulatory assets	123.4	127.7
System support resource
Regulatory assets
Total regulatory assets	105.5	113.2
Decoupling
Regulatory assets
Total regulatory assets	100.5	27.3
Securitization
Regulatory assets
Total regulatory assets	79.1	85.9
Bluewater
Regulatory assets
Total regulatory assets	54.8	45.3
Derivatives
Regulatory assets
Total regulatory assets	50.6	130.3
Energy efficiency programs
Regulatory assets
Total regulatory assets	32.4	33.9
Other, net
Regulatory assets
Total regulatory assets	$ 149.9	$ 124.5