RISK MANAGEMENT AND HEDGING ACTIVITIES USING DERIVATIVES - Part 3 (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
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	$ 27,115	$ (47,883)	$ (90,586)
Unrealized change in fair value of cash flow hedges in other comprehensive income (loss)	(191,952)	(7,367)	(40,414)
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)	38,819	53,217	(40,577)
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	50,345	(29,596)	(69,561)
Interest Rate 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)	(230,771)	(60,584)	163
Interest Rate Contracts | 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	(23,230)	(18,287)	(21,025)
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	50,345	(29,596)	(69,561)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | 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	$ (23,230)	$ (18,287)	$ (21,025)