Fair Value Measurements (Details) - Fair value measured on a recurring basis - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Fair value measurements
Cash and cash equivalents	$ 49,867	$ 96,314	$ 78,555
Available for sale securities	192,712	171,596	189,289
Trading securities	63,520	60,520	73,025
Total assets measured at fair value	305,025	328,541	340,824
Foreign currency forward contracts
Fair value measurements
Derivative instruments, net	21	79	(71)
Commodity futures contracts
Fair value measurements
Derivative instruments, net	(1,095)	32	26
Level 1
Fair value measurements
Cash and cash equivalents	49,867	96,314	78,555
Available for sale securities		1,200	2,412
Trading securities	63,520	60,520	73,025
Total assets measured at fair value	112,292	158,066	154,018
Level 1 \| Commodity futures contracts
Fair value measurements
Derivative instruments, net	(1,095)	32	26
Level 2
Fair value measurements
Available for sale securities	192,712	170,396	186,877
Total assets measured at fair value	192,733	170,475	186,806
Level 2 \| Foreign currency forward contracts
Fair value measurements
Derivative instruments, net	$ 21	$ 79	$ (71)

Amount of debt securities categorized neither as held-to-maturity nor trading.

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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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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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