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ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) ACCUMULATED OTHER COMPREHENSIVE INCOME (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Risk management assets/liabilities - January 1		$ (64,660)			$ (64,660)
Retirement and Other Postretirement Benefit Plan Obligations - January 1		(121,785)			(121,785)
Risk-management assets/liabilities of unconsolidated affiliates - January 1		(1,794)			(1,794)
Accumulated other comprehensive loss - January 1		(188,239)			(188,239)
Other Comprehensive Income (Loss) Before Reclassifications, Risk-Management Assets/Liabilities					(125,973)
Other Comprehensive (Income) Loss Before Reclassification, Retirement and Postretirement Benefit Plan Obligations					(84)
Other Comprehensive Income (Loss) Before Reclassification, Risk-Management Assets/Liabilities of Unconsolidated Affiliates					(6,122)
Other comprehensive income (loss) before reclassifications					(132,179)
Other comprehensive Income (Loss) Reclassification Adjustment from AOCI for Risk-Management Assets/Liabilities, Net of Tax	$ (8,625)		$ 12,160		(20,796)	$ 24,136
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Other Comprehensive Income Attributable to Unconsolidated Affiliates, Net of Tax					72
Other comprehensive Income (Loss), Reclassification Adjustment included in Net Income, Net of Tax	(6,194)		15,326		(15,978)	30,554
Other Comprehensive Income (Loss), Risk-Management Assets/Liabilities, after Reclassification Adjustment, after Tax					(146,769)
Other Comprehensive (Income) Loss, Retirement and Other Postretirement Benefit Plans, after Reclassification Adjustment, after Tax	2,465		3,235		4,806	5,850
Other Comprehensive Income (Loss), Risk-Management Assets/Liabilities of Unconsolidated Affiliates	(3,683)		977		(6,194)	3,801
Other Comprehensive Income (Loss), Net of Tax	(66,872)	(81,285)	(3,897)	$ 51,939	(148,157)	48,042
Risk management assets/liabilities - June 30	(211,429)				(211,429)
Retirement and Other Postretirement Benefit Plan Obligations - June 30	(116,979)				(116,979)
Risk-management assets/liabilities of unconsolidated affiliates - June 30	(7,988)				(7,988)
Accumulated other comprehensive loss - June 30	(336,396)				(336,396)
Reclassification Adjustment from AOCI on Risk Management Assets/Liabilities, Income Tax	(1,804)		3,631		(5,981)	7,209
Price Risk Cash Flow Hedge Unrealized Gain (Loss) to be Reclassified During Next 12 Months	18,600				18,600
Commodity Cash Flow Hedge Gain Loss To Be Reclassified After Next 12 Months Net	1,200				1,200
Interest Rate Cash Flow Hedge Gain (Loss) to be Reclassified During Next 12 Months, Net	13,800				13,800
Accumulated Net Gain (Loss) from Cash Flow Hedges [Member] \| Sales [Member] \| Commodity Contract [Member]
Derivative instruments, Gain (Loss) Reclassified from Accumulated OCI into Income	15,244		(10,189)		34,096	(21,800)
Accumulated Net Gain (Loss) from Cash Flow Hedges [Member] \| Interest Expense [Member] \| Interest Rate Contract [Member]
Derivative instruments, Gain (Loss) Reclassified from Accumulated OCI into Income	(4,815)		(5,602)		(7,319)	(9,545)
Accumulated Net Gain (Loss) from Cash Flow Hedges [Member] \| Total before tax [Member]
Derivative instruments, Gain (Loss) Reclassified from Accumulated OCI into Income	10,429		(15,791)		26,777	(31,345)
Accumulated Net Gain (Loss) from Cash Flow Hedges [Member] \| Tax Expense [Member]
Reclassification Adjustment from AOCI on Risk Management Assets/Liabilities, Income Tax	(1,804)		3,631		(5,981)	7,209
Accumulated Net Gain (Loss) from Cash Flow Hedges [Member] \| Net of tax [Member]
Other comprehensive Income (Loss) Reclassification Adjustment from AOCI for Risk-Management Assets/Liabilities, Net of Tax	(8,625)		12,160		(20,796)	24,136
Accumulated Retirement and Other Postretirement Benefit Plans Adjustment [Member]
Other Comprehensive (Income) Loss, Retirement and Other Postretirement Benefit Plans, Reclassification Adjustment from AOCI, After Tax	(2,445)		(3,217)		(4,890)	(6,433)
Retirement and Other Postretirement Benefit Plans, Amortization of Gain (Loss)	(3,232)		(4,592)		(6,464)	(9,184)
Retirement and Other Postretirement Benefit Plans, Amortization of Prior Service Cost (Credit)	57		415		114	830
Reclassification Adjustment from AOCI, before Tax, Retirement and Other Postretirement Benefit Plans, Gain (Loss)	(3,175)		(4,177)		(6,350)	(8,354)
Reclassification Adjustment from AOCI, Tax, Retirement and Other Postretirement Benefit Plans, Gain (Loss)	730		960		1,460	1,921
Accumulated Other Comprehensive Income from Investments in Unconsolidated Affiliates [Member]
Reclassification Adjustment from AOCI, Before Tax, Portion Attributable to Unconsolidated Affiliates	18		66		93	19
Reclassification Adjustment from AOCI, Tax, Portion Attributable to Unconsolidated Affiliates	(4)		(15)		(21)	(4)
Accumulated Other Comprehensive Income from Investments in Unconsolidated Affiliates [Member] \| Net of tax [Member]
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI for Other Comprehensive Income Attributable to Unconsolidated Affiliates, Net of Tax	14		51		72	$ 15
AOCI [Member]
Other Comprehensive Income (Loss), Net of Tax	(66,872)	$ (81,285)	$ (3,897)	$ 51,939	(148,157)
Cash Flow Hedging [Member] \| Commodity Contract [Member]
Unrealized Gain Loss On Cash Flow Hedges Net Of Tax Accumulated Other Comprehensive Income Loss	19,756				19,756
Cash Flow Hedging [Member] \| Interest Rate Contract [Member]
Amount of accumulated other comprehensive income (loss) attributable primarily to settled interest-rate swaps.	(33,341)				(33,341)
Cash Flow Hedging [Member] \| Forward Contracts [Member]
Amount of accumulated other comprehensive income (loss) attributable primarily to forward starting interest-rate swaps.	$ (197,844)				$ (197,844)