REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Regulatory assets
Other current assets	$ 87.0	$ 102.3
Regulatory assets	3,164.2	3,264.8
Total regulatory assets	3,251.2	3,367.1
Pension and OPEB costs
Regulatory assets
Total regulatory assets	742.9	802.3
Plant retirement related items
Regulatory assets
Total regulatory assets	698.0	722.3
Environmental remediation costs
Regulatory assets
Total regulatory assets	616.3	630.9
Income tax related items
Regulatory assets
Total regulatory assets	457.8	458.8
Asset retirement obligations
Regulatory assets
Total regulatory assets	181.9	194.2
System support resource
Regulatory assets
Total regulatory assets	125.5	129.5
Securitization
Regulatory assets
Total regulatory assets	94.0	100.7
Energy costs recoverable through rate adjustments
Regulatory assets
Total regulatory assets	73.5	85.4
Derivatives
Regulatory assets
Total regulatory assets	70.6	33.1
MERC extraordinary natural gas costs
Regulatory assets
Total regulatory assets	44.9	59.7
Uncollectible expense
Regulatory assets
Total regulatory assets	36.6	42.6
Energy efficiency programs
Regulatory assets
Total regulatory assets	29.6	22.0
Other, net
Regulatory assets
Total regulatory assets	$ 79.6	$ 85.6