REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Regulatory assets
Regulatory assets	$ 424.6	$ 437.9
Pension and OPEB costs
Regulatory assets
Regulatory assets	137.8	151.8
Environmental remediation costs
Regulatory assets
Regulatory assets	112.4	113.5
Plant retirements
Regulatory assets
Regulatory assets	57.9	55.3
Income tax related items
Regulatory assets
Regulatory assets	44.6	38.9
ReACT
Regulatory assets
Regulatory assets	18.8	20.8
Asset retirement obligations
Regulatory assets
Regulatory assets	16.5	8.4
Forward Wind Energy Center
Regulatory assets
Regulatory assets	12.2	17.9
Termination of a tolling agreement with Fox Energy Company LLC
Regulatory assets
Regulatory assets	5.8	9.9
Other, net
Regulatory assets
Regulatory assets	$ 18.6	$ 21.4