REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Regulatory assets
Other current assets	$ 92.0	$ 102.3
Regulatory assets	3,163.6	3,264.8
Total regulatory assets	3,255.6	3,367.1
Pension and OPEB costs
Regulatory assets
Total regulatory assets	783.1	802.3
Plant retirement related items
Regulatory assets
Total regulatory assets	714.8	722.3
Environmental remediation costs
Regulatory assets
Total regulatory assets	618.8	630.9
Income tax related items
Regulatory assets
Total regulatory assets	456.6	458.8
Asset retirement obligations
Regulatory assets
Total regulatory assets	179.6	194.2
System support resource
Regulatory assets
Total regulatory assets	128.4	129.5
Securitization
Regulatory assets
Total regulatory assets	98.2	100.7
Energy costs recoverable through rate adjustments
Regulatory assets
Total regulatory assets	70.1	85.4
MERC extraordinary natural gas costs
Regulatory assets
Total regulatory assets	51.6	59.7
Uncollectible expense
Regulatory assets
Total regulatory assets	47.6	42.6
Energy efficiency programs
Regulatory assets
Total regulatory assets	20.4	22.0
Derivatives
Regulatory assets
Total regulatory assets	3.6	33.1
Other, net
Regulatory assets
Total regulatory assets	$ 82.8	$ 85.6