DERIVATIVE INSTRUMENTS - DERIVATIVE ASSETS AND LIABILITIES (Details) $ in Millions	Jun. 30, 2025 USD ($) Instruments	Dec. 31, 2024 USD ($) Instruments
Derivative assets
Current derivative assets	$ 52.7	$ 37.0
Derivative assets	$ 56.4	$ 41.1
Current derivative assets balance sheet location	Other	Other
Long-term derivative assets balance sheet location	Other Assets, Noncurrent	Other Assets, Noncurrent
Derivative liabilities
Current derivative liabilities	$ 15.5	$ 13.9
Derivative liabilities	$ 15.8	$ 13.9
Current derivative liabilities balance sheet location	Other Liabilities, Current	Other Liabilities, Current
Long-term derivative liabilities balance sheet location	Other noncurrent liabilities	Other noncurrent liabilities
Natural gas contracts
Derivative assets
Current derivative assets	$ 33.7	$ 29.2
Long-term derivative assets	3.7	4.1
Derivative liabilities
Current derivative liabilities	15.5	13.9
Long-term derivative liabilities	0.3	0.0
FTRs and TCRs
Derivative assets
Current derivative assets	19.0	7.8
Derivative liabilities
Current derivative liabilities	$ 0.0	$ 0.0
Designated as hedging instrument
Derivative instruments
Number of derivative instruments | Instruments	0	0