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RISK MANAGEMENT AND HEDGING ACTIVITIES USING DERIVATIVES Part 3 (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Cash Flow Hedging [Member]        
Derivative Instruments, Gain (Loss) [Line Items]        
Unrealized change in fair value of cash flow hedges in other comprehensive income (loss) $ (95,872) $ (4,580) $ (259,474) $ 13,933
Change in fair value of cash flow hedges reclassified from accumulated other comprehensive loss into net income 11,625 (25,013) 38,402 (56,359)
Commodity contracts | Cash Flow Hedging [Member]        
Derivative Instruments, Gain (Loss) [Line Items]        
Unrealized change in fair value of cash flow hedges in other comprehensive income (loss) 32,169 (30,783) 43,877 (56,246)
Commodity contracts | Commodity sales revenues/ cost of sales and fuel | Cash Flow Hedging [Member]        
Derivative Instruments, Gain (Loss) [Line Items]        
Change in fair value of cash flow hedges reclassified from accumulated other comprehensive loss into net income   (20,630)   (42,430)
Interest Rate Contract [Member] | Cash Flow Hedging [Member]        
Derivative Instruments, Gain (Loss) [Line Items]        
Unrealized change in fair value of cash flow hedges in other comprehensive income (loss) (128,041) 26,203 (303,351) 70,179
Interest Rate Contract [Member] | Interest expense | Cash Flow Hedging [Member]        
Derivative Instruments, Gain (Loss) [Line Items]        
Change in fair value of cash flow hedges reclassified from accumulated other comprehensive loss into net income   (4,383)   (13,929)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Commodity contracts | Commodity sales revenues/ 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 18,528 (20,630) 52,624 (42,430)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Interest Rate Contract [Member] | 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 $ (6,903) $ (4,383) $ (14,222) $ (13,929)