Fair Value Measurements (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Fair value measurements
Cash and cash equivalents	$ 57,857	$ 110,899	$ 38,334
Fair value measured on a recurring basis
Fair value measurements
Cash and cash equivalents	57,857	110,899	38,334
Available for sale securities	206,687	183,289	192,392
Trading securities	71,375	62,260	65,000
Total assets measured at fair value	335,790	355,454	294,822
Fair value measured on a recurring basis \| Foreign currency forward contracts
Fair value measurements
Derivative instruments, net	(17)	(407)
Fair value measured on a recurring basis \| Commodity futures contracts
Fair value measurements
Derivative instruments, net	(112)	(587)	(904)
Fair value measured on a recurring basis \| Level 1
Fair value measurements
Cash and cash equivalents	57,857	110,899	38,334
Available for sale securities	3,078	3,000
Trading securities	71,375	62,260	65,000
Total assets measured at fair value	132,198	175,572	102,430
Fair value measured on a recurring basis \| Level 1 \| Commodity futures contracts
Fair value measurements
Derivative instruments, net	(112)	(587)	(904)
Fair value measured on a recurring basis \| Level 2
Fair value measurements
Available for sale securities	203,609	180,289	192,392
Total assets measured at fair value	203,592	179,882	$ 192,392
Fair value measured on a recurring basis \| Level 2 \| Foreign currency forward contracts
Fair value measurements
Derivative instruments, net	$ (17)	$ (407)

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount 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. Excludes cash and cash equivalents within disposal group and discontinued operation.

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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 investment in debt security measured at fair value with change in fair value recognized in net income (trading) and investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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