REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Regulatory assets
Regulatory assets	$ 449.4	$ 437.9
Allowance for return on equity capitalized for regulatory purposes	16.6	20.8
Regulatory assets not earning a return	39.7
Estimated future cash expenditures for environmental remediation	88.3	83.8
Pension and OPEB costs
Regulatory assets
Regulatory assets	161.4	151.8
Environmental remediation costs
Regulatory assets
Regulatory assets	116.1	113.5
Cash expenditures for environmental remediation costs	27.8
Estimated future cash expenditures for environmental remediation	88.3
Plant retirements
Regulatory assets
Regulatory assets	58.5	55.3
Income tax related items
Regulatory assets
Regulatory assets	46.7	38.9
ReACT
Regulatory assets
Regulatory assets	18.2	20.8
Asset retirement obligations (AROs)
Regulatory assets
Regulatory assets	13.1	8.4
Uncollectible expense
Regulatory assets
Regulatory assets	12.5	4.2
Forward Wind Energy Center
Regulatory assets
Regulatory assets	$ 10.3	17.9
Recovery period of regulatory asset	2 years
Other, net
Regulatory assets
Regulatory assets	$ 12.6	$ 27.1