ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Risk management assets/liabilities - January 1			$ (233,520)				$ (233,520)
Retirement and other postretirement benefit plan obligations - January 1			(131,481)				(131,481)
Risk-management assets/liabilities of unconsolidated affiliates - January 1			(8,999)				(8,999)
Accumulated other comprehensive loss - January 1			(374,000)				(374,000)
Other comprehensive income (loss) before reclassification, risk-management assets/liabilities	$ (15,750)			$ (73,822)			(168,421)	$ (199,795)
Other comprehensive (income) loss before reclassification, retirement and postretirement benefit plan obligations							59
Other comprehensive income (loss) before reclassification, risk-management assets/liabilities of unconsolidated affiliates							(10,893)
Other comprehensive income (loss) before reclassifications							(179,255)
Other comprehensive income reclassification adjustment from AOCI for risk-management assets/liabilities, net of tax	5,361			(8,952)			11,210	(29,748)
Other comprehensive income, retirement and other postretirement benefit plans, reclassification adjustment from AOCI, net of tax							10,626
Other comprehensive income, reclassification adjustment from AOCI for other comprehensive income attributable to unconsolidated affiliates, net of tax							801
Other comprehensive income, reclassification adjustment included in net income, net of tax							22,637
Other comprehensive income (loss), risk-management assets/liabilities, after reclassification adjustment, net of tax							(157,211)
Other comprehensive (income) loss, retirement and other postretirement benefit plans, after reclassification adjustment, net of tax	3,562			2,465			10,685	7,271
Other comprehensive income (loss), risk-management assets/liabilities of unconsolidated affiliates, net of tax	1,112			(4,399)			(10,092)	(10,593)
Other comprehensive income (loss), net of tax	(5,715)	$ (25,517)	$ (125,386)	$ (84,708)	$ (66,872)	$ (81,285)	(156,618)	$ (232,865)
Risk management assets/liabilities - September 30	(390,731)						(390,731)
Retirement and other postretirement benefit plan obligations - September 30	(120,796)						(120,796)
Risk-management assets/liabilities of unconsolidated affiliates - September 30	(19,091)						(19,091)
Accumulated other comprehensive loss - September 30	(530,618)						(530,618)
Cash Flow Hedging [Member] | Commodity contracts
Unrealized gain loss on cash flow hedges net of tax accumulated other comprehensive income loss	(10,988)						(10,988)
Price risk cash flow hedge unrealized gain (loss) to be reclassified during next 12 months	(8,400)						(8,400)
Commodity cash flow hedge gain (loss) to be reclassified after next 12 months net	(2,600)						(2,600)
Cash Flow Hedging [Member] | Interest-rate contracts
Amount of accumulated other comprehensive income (loss) attributable primarily to settled interest-rate swaps.	(201,110)						(201,110)
Amount of accumulated other comprehensive income (loss) attributable primarily to forward starting interest-rate swaps.	(178,633)						(178,633)
Interest rate cash flow hedge gain (loss) to be reclassified during next 12 months, net	$ (30,300)						$ (30,300)