REGULATORY ASSETS AND LIABILITIES - REGULATORY LIABILITIES (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Regulatory liabilities
Other current liabilities	$ 9.4	$ 0.3
Regulatory liabilities	698.8	696.3
Total regulatory liabilities	708.2	696.6
Income tax related items
Regulatory liabilities
Total regulatory liabilities	314.2	322.0
Removal costs
Regulatory liabilities
Total regulatory liabilities	210.7	206.6
Pension and OPEB benefits
Regulatory liabilities
Total regulatory liabilities	86.0	88.6
Energy costs refundable through rate adjustments
Regulatory liabilities
Total regulatory liabilities	61.6	49.2
Derivatives
Regulatory liabilities
Total regulatory liabilities	13.4	8.0
Other, net
Regulatory liabilities
Total regulatory liabilities	$ 22.3	$ 22.2