REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Millions	Sep. 30, 2023	Dec. 31, 2022
Regulatory assets
Other current assets	$ 15.8	$ 42.3
Regulatory assets	3,197.1	3,264.6
Total regulatory assets	3,212.9	3,306.9
Pension and OPEB costs
Regulatory assets
Total regulatory assets	691.2	714.3
Plant retirement related items
Regulatory assets
Total regulatory assets	657.0	688.6
Environmental remediation costs
Regulatory assets
Total regulatory assets	577.7	610.7
Income tax related items
Regulatory assets
Total regulatory assets	450.4	461.9
Asset retirement obligations
Regulatory assets
Total regulatory assets	170.1	169.7
Derivatives
Regulatory assets
Total regulatory assets	126.2	133.8
System support resource
Regulatory assets
Total regulatory assets	115.9	123.5
Uncollectible expense
Regulatory assets
Total regulatory assets	95.5	69.3
Securitization
Regulatory assets
Total regulatory assets	87.5	92.4
Bluewater
Regulatory assets
Total regulatory assets	39.9	20.9
Energy costs recoverable through rate adjustments
Regulatory assets
Total regulatory assets	35.5	26.9
Energy efficiency programs
Regulatory assets
Total regulatory assets	25.3	33.9
MERC extraordinary natural gas costs
Regulatory assets
Total regulatory assets	9.3	35.1
Other, net
Regulatory assets
Total regulatory assets	$ 131.4	$ 125.9