Derivative Financial Instruments (Narrative) (Details) - Futures - Commodity contract - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Other current assets
Commodity contract commitments
Fair value of derivatives including required margin deposits	$ 3,500		$ 5,600
Designated as hedging instrument
Gains or losses on derivative instruments, statement of income
Ineffectiveness - amount of gain (loss) recognized in income on related hedged items	(101)	$ 48
Gain (loss) excluded from hedge effectiveness testing	$ 5,000	$ 833

Fair value, after the effects of master netting arrangements, of a 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. Includes assets not subject to a master netting arrangement and not elected to be offset.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Net gain (loss) relating to components of the gain (loss) on the fair value hedging instrument excluded from the assessment of fair value hedge effectiveness. Recognized in earnings.

+ References

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Amount of gain (loss)recognized in the income statement from the hedging ineffectiveness of a fair value hedge.

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