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RISK MANAGEMENT AND HEDGING ACTIVITIES USING DERIVATIVES - Part 3 (Details) - Cash Flow Hedging [Member] - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Derivative Instruments, Gain (Loss) [Line Items]      
Unrealized change in fair value of cash flow hedges in other comprehensive income (loss) $ (214,315) $ (191,952) $ (7,367)
Change in fair value of cash flow hedges reclassified from accumulated other comprehensive loss into net income on derivatives (27,410) 27,115 (47,883)
Commodity Contract      
Derivative Instruments, Gain (Loss) [Line Items]      
Unrealized change in fair value of cash flow hedges in other comprehensive income (loss) (5,699) 38,819 53,217
Commodity Contract | Commodity sales revenues      
Derivative Instruments, Gain (Loss) [Line Items]      
Change in fair value of cash flow hedges reclassified from accumulated other comprehensive loss into net income on derivatives 85,436 94,547 (37,596)
Commodity Contract | Cost of sales and fuel      
Derivative Instruments, Gain (Loss) [Line Items]      
Change in fair value of cash flow hedges reclassified from accumulated other comprehensive loss into net income on derivatives (19,170) (44,202) 8,000
Interest Rate Contract      
Derivative Instruments, Gain (Loss) [Line Items]      
Unrealized change in fair value of cash flow hedges in other comprehensive income (loss) (208,616) (230,771) (60,584)
Interest Rate Contract | Interest expense      
Derivative Instruments, Gain (Loss) [Line Items]      
Change in fair value of cash flow hedges reclassified from accumulated other comprehensive loss into net income on derivatives $ (93,676) $ (23,230) $ (18,287)