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RISK MANAGEMENT AND HEDGING ACTIVITIES USING DERIVATIVES Part 3 (Details) - Cash Flow Hedging [Member] - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Derivative Instruments, Gain (Loss) [Line Items]        
Unrealized change in fair value of cash flow hedges in other comprehensive income (loss) $ (60,428) $ (74,064) $ (198,274) $ (163,601)
Change in fair value of cash flow hedges reclassified from accumulated other comprehensive loss into net income (loss) on derivatives (25,853) 10,429 (6,171) 26,777
Commodity contracts        
Derivative Instruments, Gain (Loss) [Line Items]        
Unrealized change in fair value of cash flow hedges in other comprehensive income (loss) (39,056) 33,334 48,139 11,709
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 53,138 23,319 98,137 47,141
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 (14,450) (8,075) (29,489) (13,045)
Interest-rate contracts        
Derivative Instruments, Gain (Loss) [Line Items]        
Unrealized change in fair value of cash flow hedges in other comprehensive income (loss) (21,372) (107,398) (246,413) (175,310)
Interest-rate contracts | 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 $ (64,541) $ (4,815) $ (74,819) $ (7,319)