Derivative Financial Instruments (Schedule of Derivatives Instruments Statements of Financial Performance and Financial Position) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Commodity futures, asset derivatives	$ 4,558	$ 2,790
Commodity futures, liability derivatives	3,890	10,506
Futures \| Commodity contract \| Designated as hedging instrument \| Other current assets
Commodity futures, asset derivatives	2,999	1,211
Commodity futures, liability derivatives	1,837	5,364
Futures \| Commodity contract \| Not designated as hedging instrument \| Other current assets
Commodity futures, asset derivatives	1,559	1,579
Commodity futures, liability derivatives	$ 2,053	$ 5,142

Fair value, before effects of master netting arrangements, of a financial liability or 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. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4B
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
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Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226000-175313

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