REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Millions	Jun. 30, 2024	Dec. 31, 2023
Regulatory assets
Regulatory assets	$ 456.6	$ 360.6
Plant retirement related items
Regulatory assets
Regulatory assets	137.2	38.4
Plant retirement related items | Coal Combustion Residuals Rule
Regulatory assets
Regulatory assets	96.5
Environmental remediation costs
Regulatory assets
Regulatory assets	119.1	121.5
Pension and OPEB costs
Regulatory assets
Regulatory assets	63.2	62.0
Income tax related items
Regulatory assets
Regulatory assets	55.9	57.2
AROs
Regulatory assets
Regulatory assets	18.9	18.8
Bluewater Natural Gas Holding, LLC
Regulatory assets
Regulatory assets	13.1	11.9
ReACT
Regulatory assets
Regulatory assets	9.1	10.4
Uncollectible expense
Regulatory assets
Regulatory assets	8.2	8.9
Derivatives
Regulatory assets
Regulatory assets	5.0	12.5
Other, net
Regulatory assets
Regulatory assets	$ 26.9	$ 19.0