Fair Value Measurements (Details) (Fair value measured on a recurring basis, USD $) In Thousands, unless otherwise specified	Mar. 30, 2013	Dec. 31, 2012	Mar. 31, 2012
Estimated Fair Value
Fair value measurements
Cash and cash equivalents	$ 50,887	$ 63,862	$ 79,909
Auction rate security	8,808	9,485	6,775
Available-for-sale securities excluding the auction rate security	116,771	84,568	59,054
Trading securities	54,377	49,378	47,031
Total assets measured at fair value	230,147	207,181	193,550
Estimated Fair Value \| Foreign currency forward contracts
Fair value measurements
Derivative instruments, net	(247)		308
Estimated Fair Value \| Commodity futures contracts, net
Fair value measurements
Derivative instruments, net	(449)	(112)	440
Estimated Fair Value \| Commodity option contracts
Fair value measurements
Derivative instruments, net			33
Level 1
Fair value measurements
Cash and cash equivalents	50,887	63,862	79,909
Trading securities	54,377	49,378	47,031
Total assets measured at fair value	104,815	113,128	127,413
Level 1 \| Commodity futures contracts, net
Fair value measurements
Derivative instruments, net	(449)	(112)	440
Level 1 \| Commodity option contracts
Fair value measurements
Derivative instruments, net			33
Level 2
Fair value measurements
Available-for-sale securities excluding the auction rate security	116,771	84,568	59,054
Total assets measured at fair value	116,524	84,568	59,362
Level 2 \| Foreign currency forward contracts
Fair value measurements
Derivative instruments, net	(247)		308
Level 3
Fair value measurements
Auction rate security	8,808	9,485	6,775
Total assets measured at fair value	$ 8,808	$ 9,485	$ 6,775

This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Company may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months.

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This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Trading Securities which consist of all investments in certain debt and equity securities that are bought and held principally for the purpose of selling them in the near term (thus held for only a short period of time) or for which an entity has elected the fair value option. Trading generally reflects active and frequent buying and selling, and trading securities are generally used with the objective of generating profits on short-term differences in price, but this category of investments may also include items which are not intended to be sold in the near term or for which the fair value option has been elected.

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