Fair Value Measurements (Details) - Fair value measured on a recurring basis - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016
Fair value measurements
Cash and cash equivalents	$ 57,804	$ 119,145	$ 91,950
Available for sale securities	192,983	164,183	156,051
Trading securities	75,265	67,995	63,866
Total assets measured at fair value	325,787	352,950	313,617
Foreign currency forward contracts
Fair value measurements
Derivative instruments, net	83	(119)	(911)
Commodity futures contracts
Fair value measurements
Derivative instruments, net	(348)	1,746	2,661
Level 1
Fair value measurements
Cash and cash equivalents	57,804	119,145	91,950
Available for sale securities	2,406	2,419	2,440
Trading securities	75,265	67,995	63,866
Total assets measured at fair value	135,127	191,305	160,917
Level 1 \| Commodity futures contracts
Fair value measurements
Derivative instruments, net	(348)	1,746	2,661
Level 2
Fair value measurements
Available for sale securities	190,577	161,764	153,611
Total assets measured at fair value	190,660	161,645	152,700
Level 2 \| Foreign currency forward contracts
Fair value measurements
Derivative instruments, net	$ 83	$ (119)	$ (911)

Amount of investment in debt and equity 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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
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Fair value of the assets less the liabilities of a derivative or group of derivatives.

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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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