Investments (Details) (USD $)	Jun. 29, 2013	Dec. 31, 2012
Investments
Total available-for-sale and trading short term investments, amortized cost	$ 358,191,000	$ 308,064,000
Total available-for-sale and trading short term investments, fair value	358,506,000	313,379,000
Estimated fair value of fixed rate securities designated as available-for-sale, classified by the contractual maturity date of the security
Due within one year	2,128,000
Due after one year through three years	57,362,000
Due after three years	51,283,000
Total fixed rate securities	110,773,000
Available-for-sale securities
Investments
Amortized Cost	328,531,000	279,556,000
Fair Value	328,281,000	282,829,000
Money market funds
Investments
Amortized Cost	148,983,000	126,537,000
Fair Value	148,983,000	126,537,000
Money market funds \| Denominated in Canadian dollars
Investments
Fair Value	7,987,000
Money market funds \| Denominated in British pounds
Investments
Fair Value	5,298,000
Money market funds \| Denominated in Euros
Investments
Fair Value	3,500,000
Corporate bonds
Investments
Amortized Cost	69,724,000	67,275,000
Fair Value	70,098,000	69,214,000
Enhanced cash mutual fund
Investments
Amortized Cost	25,000,000
Fair Value	24,978,000
U.S. Government agency securities
Investments
Amortized Cost	23,404,000	23,647,000
Fair Value	23,271,000	23,775,000
Emerging markets debt mutual fund
Investments
Amortized Cost	17,693,000	17,693,000
Fair Value	17,278,000	18,734,000
U.S. Treasury securities
Investments
Amortized Cost	16,973,000	17,165,000
Fair Value	16,935,000	17,169,000
Collateralized mortgage obligations
Investments
Amortized Cost	15,359,000	15,059,000
Fair Value	15,315,000	15,162,000
Asset backed debt securities
Investments
Amortized Cost	11,395,000	12,180,000
Fair Value	11,423,000	12,238,000
High yield trading debt securities
Investments
Amortized Cost	23,536,000	21,839,000
Fair Value	24,214,000	23,406,000
Emerging markets trading debt mutual fund
Investments
Amortized Cost	3,120,000	3,046,000
Fair Value	2,929,000	3,237,000
Emerging markets trading debt securities
Investments
Amortized Cost	1,989,000	2,361,000
Fair Value	2,034,000	2,600,000
Other trading investments
Investments
Amortized Cost	1,015,000	1,262,000
Fair Value	$ 1,048,000	$ 1,307,000

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Represents the fair value of debt securities which are expected to mature after one year and through three years from the balance sheet date and which are categorized neither as held-to-maturity nor trading securities.

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Amount of available-for-sale debt securities at fair value maturing in the next fiscal year following the latest fiscal year.

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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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 Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale.

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Investments which are intended to be sold in the short term (usually less than one year or the normal operating cycle, whichever is longer) including trading securities, available-for-sale securities, held-to-maturity securities, and other short-term investments not otherwise listed in the existing taxonomy.

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