Fair Value Measurements (Details) - Fair value measured on a recurring basis - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Fair value measurements
Cash and cash equivalents	$ 49,512	$ 119,145	$ 94,239
Available for sale securities	193,169	164,183	151,776
Trading securities	75,094	67,995	66,085
Total assets measured at fair value	317,558	352,950	314,802
Foreign currency forward contracts
Fair value measurements
Derivative instruments, net	147	(119)	(532)
Commodity futures contracts
Fair value measurements
Derivative instruments, net	(364)	1,746	3,234
Level 1
Fair value measurements
Cash and cash equivalents	49,512	119,145	94,239
Available for sale securities	2,402	2,419	2,429
Trading securities	75,094	67,995	66,085
Total assets measured at fair value	126,644	191,305	165,987
Level 1 \| Commodity futures contracts
Fair value measurements
Derivative instruments, net	(364)	1,746	3,234
Level 2
Fair value measurements
Available for sale securities	190,767	161,764	149,347
Total assets measured at fair value	190,914	161,645	148,815
Level 2 \| Foreign currency forward contracts
Fair value measurements
Derivative instruments, net	$ 147	$ (119)	$ (532)

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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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/presentationRef
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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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Fair value of asset after deduction of liability.

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Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading.

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