REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Regulatory assets
Regulatory assets	$ 414.8	$ 449.4
Pension and OPEB costs
Regulatory assets
Regulatory assets	145.9	161.4
Environmental remediation costs
Regulatory assets
Regulatory assets	113.0	116.1
Income tax related items
Regulatory assets
Regulatory assets	53.7	46.7
Plant retirements
Regulatory assets
Regulatory assets	52.4	58.5
ReACT
Regulatory assets
Regulatory assets	16.2	18.2
AROs
Regulatory assets
Regulatory assets	14.0	13.1
Uncollectible expense
Regulatory assets
Regulatory assets	4.7	12.5
Forward Wind Energy Center
Regulatory assets
Regulatory assets	4.7	10.3
Other, net
Regulatory assets
Regulatory assets	$ 10.2	$ 12.6