DERIVATIVE INSTRUMENTS - DERIVATIVE ASSETS AND LIABILITIES (Details) $ in Millions	Jun. 30, 2025 USD ($) Instruments	Dec. 31, 2024 USD ($) Instruments
Derivative assets
Current derivative assets	$ 13.4	$ 7.8
Derivative assets	$ 13.5	$ 8.1
Current derivative assets balance sheet location	Other	Other
Long-term derivative assets balance sheet location	Other	Other
Derivative liabilities
Current derivative liabilities	$ 2.3	$ 0.9
Derivative liabilities	$ 2.3	$ 0.9
Current derivative liability balance sheet location	Other	Other
Long-term derivative liabilities balance sheet location	Other	Other
Natural gas contracts
Derivative assets
Current derivative assets	$ 7.1	$ 5.8
Long-term derivative assets	0.1	0.3
Derivative liabilities
Current derivative liabilities	2.3	0.9
Long-term derivative liabilities	0.0	0.0
FTRs
Derivative assets
Current derivative assets	6.3	2.0
Derivative liabilities
Current derivative liabilities	$ 0.0	$ 0.0
Derivatives designated as hedging instruments
Derivative instruments
Number of derivative instruments | Instruments	0	0