REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Regulatory assets
Current assets	$ 0.0	$ 0.3
Regulatory assets	437.9	485.6
Total regulatory assets	437.9	485.9
Allowance for return on equity capitalized for regulatory purposes	20.8	14.7
Regulatory assets not earning a return	46.2
Estimated future cash expenditures for environmental remediation	83.8	90.3
Pension and OPEB costs
Regulatory assets
Total regulatory assets	151.8	189.8
Environmental remediation costs
Regulatory assets
Total regulatory assets	113.5	108.3
Cash expenditures for environmental remediation costs	29.7
Estimated future cash expenditures for environmental remediation	83.8
Plant retirements
Regulatory assets
Total regulatory assets	55.3	78.1
Income tax related items
Regulatory assets
Total regulatory assets	38.9	38.1
ReACT
Regulatory assets
Total regulatory assets	20.8	15.3
Forward wind energy center
Regulatory assets
Total regulatory assets	$ 17.9	8.7
Authorized recovery period	2 years
Termination of a tolling agreement with Fox Energy Company LLC
Regulatory assets
Total regulatory assets	$ 9.9	21.7
Authorized recovery period	2 years
Asset retirement obligations (AROs)
Regulatory assets
Total regulatory assets	$ 8.4	11.5
Other, net
Regulatory assets
Total regulatory assets	$ 21.4	$ 14.4