Fair Value Measurements (Details) - Fair value measured on a recurring basis - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Fair value measurements
Cash and cash equivalents	$ 95,057	$ 126,145	$ 85,430
Available for sale securities	152,724	134,501	133,412
Trading securities	62,509	60,584	75,171
Total assets measured at fair value	309,509	318,875	288,976
Foreign currency forward contracts
Fair value measurements
Derivative instruments, net	(1,350)	(2,626)	(3,328)
Commodity futures contracts
Fair value measurements
Derivative instruments, net	569	271	(1,709)
Level 1
Fair value measurements
Cash and cash equivalents	95,057	126,145	85,430
Available for sale securities	2,441	2,430	2,461
Trading securities	62,509	60,584	75,171
Total assets measured at fair value	160,576	189,430	161,353
Level 1 \| Commodity futures contracts
Fair value measurements
Derivative instruments, net	569	271	(1,709)
Level 2
Fair value measurements
Available for sale securities	150,283	132,071	130,951
Total assets measured at fair value	148,933	129,445	127,623
Level 2 \| Foreign currency forward contracts
Fair value measurements
Derivative instruments, net	$ (1,350)	$ (2,626)	$ (3,328)

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
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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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