Investments (Details) (USD $)	Sep. 28, 2013	Dec. 31, 2012
Investments
Total available-for-sale and trading short term investments, amortized cost	$ 244,481,000	$ 308,064,000
Total available-for-sale and trading short term investments, fair value	244,995,000	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	658,000
Due after one year through three years	41,940,000
Due after three years	57,131,000
Total fixed rate securities	99,729,000
Available-for-sale securities
Investments
Amortized Cost	214,408,000	279,556,000
Fair Value	214,349,000	282,829,000
Money market funds
Investments
Amortized Cost	78,435,000	126,537,000
Fair Value	78,435,000	126,537,000
Money market funds | Denominated in Euros
Investments
Fair Value	15,898,000
Money market funds | Denominated in British pounds
Investments
Fair Value	7,680,000
Money market funds | Denominated in Canadian dollars
Investments
Fair Value	4,130,000
Corporate bonds
Investments
Amortized Cost	68,411,000	67,275,000
Fair Value	69,038,000	69,214,000
U.S. Government agency securities
Investments
Amortized Cost	20,627,000	23,647,000
Fair Value	20,412,000	23,775,000
Emerging markets debt mutual fund
Investments
Amortized Cost	17,693,000	17,693,000
Fair Value	17,253,000	18,734,000
Collateralized mortgage obligations
Investments
Amortized Cost	15,058,000	15,059,000
Fair Value	15,007,000	15,162,000
Asset backed debt securities
Investments
Amortized Cost	8,926,000	12,180,000
Fair Value	8,967,000	12,238,000
U.S. Treasury securities
Investments
Amortized Cost	5,258,000	17,165,000
Fair Value	5,237,000	17,169,000
High yield trading debt securities
Investments
Amortized Cost	24,445,000	21,839,000
Fair Value	25,195,000	23,406,000
Emerging markets trading debt mutual fund
Investments
Amortized Cost	3,161,000	3,046,000
Fair Value	2,900,000	3,237,000
Emerging markets trading debt securities
Investments
Amortized Cost	1,493,000	2,361,000
Fair Value	1,535,000	2,600,000
Other trading investments
Investments
Amortized Cost	974,000	1,262,000
Fair Value	$ 1,016,000	$ 1,307,000