REGULATORY ASSETS AND LIABILITIES - REGULATORY ASSETS (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Regulatory assets
Current assets	$ 0.0	$ 0.3
Regulatory assets	485.6	485.6
Total regulatory assets	485.6	485.9
Pension and OPEB costs
Regulatory assets
Total regulatory assets	183.3	189.8
Environmental remediation costs
Regulatory assets
Total regulatory assets	125.2	108.3
Plant retirements
Regulatory assets
Total regulatory assets	76.6	78.1
Income tax related items
Regulatory assets
Total regulatory assets	38.1	38.1
Asset retirement obligations
Regulatory assets
Total regulatory assets	21.1	11.5
Termination of a tolling agreement with Fox Energy Company LLC
Regulatory assets
Total regulatory assets	16.3	21.7
De Pere Energy Center
Regulatory assets
Total regulatory assets	5.2	10.1
Other, net
Regulatory assets
Total regulatory assets	$ 19.8	$ 28.3