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RISK MANAGEMENT AND HEDGING ACTIVITIES USING DERIVATIVES Part 3 (Details) - Cash Flow Hedging [Member] - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Derivative Instruments, Gain (Loss) [Line Items]    
Unrealized change in fair value of cash flow hedges in other comprehensive income (loss) $ (137,846) $ (89,537)
Change in fair value of cash flow hedges reclassified from accumulated other comprehensive loss into net income (loss) on derivatives 19,682 16,348
Commodity contracts    
Derivative Instruments, Gain (Loss) [Line Items]    
Unrealized change in fair value of cash flow hedges in other comprehensive income (loss) 87,195 (21,625)
Commodity contracts | 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 (loss) on derivatives 44,999 23,822
Commodity contracts | 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 (loss) on derivatives (15,039) (4,970)
Interest Rate Contract    
Derivative Instruments, Gain (Loss) [Line Items]    
Unrealized change in fair value of cash flow hedges in other comprehensive income (loss) (225,041) (67,912)
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 (loss) on derivatives $ (10,278) $ (2,504)