DERIVATIVE INSTRUMENTS AND HEDGING ACTIVITIES - Fair Value of Each Classification of Derivative Instrument (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Derivatives, Fair Value
Fair value	$ 4,744	$ 17,699
Netting balance sheet adjustment	0	0
Net Total
Derivatives, Fair Value
Fair value	4,744	17,699
Total current derivative assets
Derivatives, Fair Value
Fair value	5,917	18,220
Netting balance sheet adjustment	(266)	(201)
Total current derivative assets \| Physical fixed-price derivative contracts
Derivatives, Fair Value
Fair value	5,778	17,934
Netting balance sheet adjustment	(230)	(84)
Total current derivative assets \| Physical index derivative contracts
Derivatives, Fair Value
Fair value	139	286
Netting balance sheet adjustment	(36)	(117)
Total current derivative assets \| Net Total
Derivatives, Fair Value
Fair value	5,651	18,019
Total current derivative assets \| Net Total \| Physical fixed-price derivative contracts
Derivatives, Fair Value
Fair value	5,548	17,850
Total current derivative assets \| Net Total \| Physical index derivative contracts
Derivatives, Fair Value
Fair value	103	169
Total non-current derivative assets
Derivatives, Fair Value
Fair value	138	1,003
Netting balance sheet adjustment	0	0
Total non-current derivative assets \| Physical fixed-price derivative contracts
Derivatives, Fair Value
Fair value	138	1,003
Netting balance sheet adjustment	0	0
Total non-current derivative assets \| Net Total
Derivatives, Fair Value
Fair value	138	1,003
Total non-current derivative assets \| Net Total \| Physical fixed-price derivative contracts
Derivatives, Fair Value
Fair value	138	1,003
Total current derivative liabilities
Derivatives, Fair Value
Fair value	(1,311)	(1,521)
Netting balance sheet adjustment	266	201
Total current derivative liabilities \| Physical fixed-price derivative contracts
Derivatives, Fair Value
Fair value	(1,166)	(1,386)
Netting balance sheet adjustment	230	84
Total current derivative liabilities \| Physical index derivative contracts
Derivatives, Fair Value
Fair value	(145)	(135)
Netting balance sheet adjustment	36	117
Total current derivative liabilities \| Net Total
Derivatives, Fair Value
Fair value	(1,045)	(1,320)
Total current derivative liabilities \| Net Total \| Physical fixed-price derivative contracts
Derivatives, Fair Value
Fair value	(936)	(1,302)
Total current derivative liabilities \| Net Total \| Physical index derivative contracts
Derivatives, Fair Value
Fair value	$ (109)	(18)
Total non-current derivative liabilities
Derivatives, Fair Value
Fair value		(3)
Netting balance sheet adjustment		0
Total non-current derivative liabilities \| Physical fixed-price derivative contracts
Derivatives, Fair Value
Fair value		(3)
Netting balance sheet adjustment		0
Total non-current derivative liabilities \| Net Total
Derivatives, Fair Value
Fair value		(3)
Total non-current derivative liabilities \| Net Total \| Physical fixed-price derivative contracts
Derivatives, Fair Value
Fair value		$ (3)

Amount of obligation to return cash collateral under master netting arrangements offset against derivative assets.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (b)
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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Fair value of liability associated with financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, and obligation to return cash collateral under master netting arrangements.

+ References

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Fair value of the assets less the liabilities of a derivative or group of derivatives.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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