Fair Value Measurements (Details) (Fair value measured on a recurring basis, USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Apr. 02, 2011
Estimated Fair Value
Fair value measurements
Cash and cash equivalents	$ 79,909	$ 78,612	$ 84,908
Auction rate security	6,775	7,453	6,775
Available-for-sale securities excluding the auction rate security	59,054	57,835	44,483
Trading securities	47,031	41,768	42,536
Total assets measured at fair value	193,550	186,076	181,962
Estimated Fair Value \| Foreign currency forward contracts
Fair value measurements
Derivative instruments, net	308	205	600
Estimated Fair Value \| Commodity futures contracts
Fair value measurements
Derivative instruments, net	440	203	373
Estimated Fair Value \| Commodity option contracts
Fair value measurements
Derivative instruments, net	33		2,287
Level 1
Fair value measurements
Cash and cash equivalents	79,909	78,612	84,908
Trading securities	47,031	41,768	42,536
Total assets measured at fair value	127,413	120,583	130,104
Level 1 \| Commodity futures contracts
Fair value measurements
Derivative instruments, net	440	203	373
Level 1 \| Commodity option contracts
Fair value measurements
Derivative instruments, net	33		2,287
Level 2
Fair value measurements
Available-for-sale securities excluding the auction rate security	59,054	57,835	44,483
Total assets measured at fair value	59,362	58,040	45,083
Level 2 \| Foreign currency forward contracts
Fair value measurements
Derivative instruments, net	308	205	600
Level 3
Fair value measurements
Auction rate security	6,775	7,453	6,775
Total assets measured at fair value	$ 6,775	$ 7,453	$ 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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