REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Millions	Jun. 30, 2023	Dec. 31, 2022
Regulatory assets
Other current assets	$ 8.6	$ 42.3
Regulatory assets	3,238.9	3,264.6
Total regulatory assets	3,247.5	3,306.9
Pension and OPEB costs
Regulatory assets
Total regulatory assets	698.8	714.3
Plant retirement related items
Regulatory assets
Total regulatory assets	667.2	688.6
Environmental remediation costs
Regulatory assets
Total regulatory assets	582.7	610.7
Income tax related items
Regulatory assets
Total regulatory assets	454.9	461.9
Asset retirement obligations
Regulatory assets
Total regulatory assets	170.9	169.7
Derivatives
Regulatory assets
Total regulatory assets	161.1	133.8
System support resource
Regulatory assets
Total regulatory assets	118.3	123.5
Securitization
Regulatory assets
Total regulatory assets	89.5	92.4
Uncollectible expense
Regulatory assets
Total regulatory assets	84.0	69.3
Bluewater
Regulatory assets
Total regulatory assets	34.9	20.9
Energy efficiency programs
Regulatory assets
Total regulatory assets	26.6	33.9
MERC extraordinary natural gas costs
Regulatory assets
Total regulatory assets	13.4	35.1
Other, net
Regulatory assets
Total regulatory assets	$ 145.2	$ 152.8