Fair Value Measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Fair value measurements
Cash and cash equivalents	$ 77,670	$ 110,899	$ 49,867
Fair value measured on a recurring basis
Fair value measurements
Cash and cash equivalents	77,670	110,899	49,867
Available for sale securities	199,944	183,289	192,712
Trading securities	69,260	62,260	63,520
Total assets measured at fair value	346,526	355,454	305,025
Fair value measured on a recurring basis \| Foreign currency forward contracts
Fair value measurements
Derivative instruments, net	(197)	(407)	21
Fair value measured on a recurring basis \| Commodity futures contracts
Fair value measurements
Derivative instruments, net	(151)	(587)	(1,095)
Fair value measured on a recurring basis \| Level 1
Fair value measurements
Cash and cash equivalents	77,670	110,899	49,867
Available for sale securities	3,041	3,000
Trading securities	69,260	62,260	63,520
Total assets measured at fair value	149,820	175,572	112,292
Fair value measured on a recurring basis \| Level 1 \| Commodity futures contracts
Fair value measurements
Derivative instruments, net	(151)	(587)	(1,095)
Fair value measured on a recurring basis \| Level 2
Fair value measurements
Available for sale securities	196,903	180,289	192,712
Total assets measured at fair value	196,706	179,882	192,733
Fair value measured on a recurring basis \| Level 2 \| Foreign currency forward contracts
Fair value measurements
Derivative instruments, net	$ (197)	$ (407)	$ 21

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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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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