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OPERATIONS AND SIGNIFICANT ACCOUNTING POLICIES - Inventories (Details) - USD ($) $ in Millions | Mar. 31, 2025 | Dec. 31, 2024
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Organization, Consolidation and Presentation of Financial Statements [Abstract] |  |
Fuel | $ 22.3 | $ 22.5
Materials and Supplies | 122.2 | 107.6
Renewable Energy Facilities Under Development | 26.6 | 24.5
Inventories – Net | $ 171.1 | $ 154.6

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