Investments (Details) (USD $)	Jun. 28, 2014	Dec. 31, 2013
Investments
Total available-for-sale and trading short term investments, amortized cost	$ 323,181,000	$ 290,042,000
Total available-for-sale and trading short term investments, fair value	325,134,000	290,649,000
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	1,963,000
Due after one year through three years	45,889,000
Due after three years	49,675,000
Total fixed rate securities	97,527,000
Available-for-sale securities
Investments
Amortized Cost	181,319,000	224,314,000
Fair Value	181,953,000	224,076,000
Money market funds
Investments
Amortized Cost	66,993,000	88,430,000
Fair Value	66,993,000	88,430,000
Money market funds | Denominated in Euros
Investments
Fair Value	6,848,000
Money market funds | Denominated in Canadian dollars
Investments
Fair Value	5,549,000
Corporate bonds
Investments
Amortized Cost	49,306,000	69,591,000
Fair Value	49,921,000	70,258,000
U.S. Government agency securities
Investments
Amortized Cost	43,615,000	27,299,000
Fair Value	43,629,000	27,147,000
Asset backed debt securities
Investments
Amortized Cost	10,791,000	8,446,000
Fair Value	10,791,000	8,477,000
Collateralized mortgage obligations
Investments
Amortized Cost	5,575,000	7,597,000
Fair Value	5,584,000	7,600,000
U.S. Treasury securities
Investments
Amortized Cost	5,039,000	5,258,000
Fair Value	5,035,000	5,223,000
Emerging markets debt mutual fund
Investments
Amortized Cost		17,693,000
Fair Value		16,941,000
High yield trading debt securities
Investments
Amortized Cost	101,802,000	49,352,000
Fair Value	103,163,000	50,428,000
Equity mutual fund
Investments
Amortized Cost	25,087,000
Fair Value	25,017,000
Money market funds held in trading accounts
Investments
Amortized Cost	5,728,000	11,033,000
Fair Value	5,728,000	11,033,000
Domestic equity ETF
Investments
Amortized Cost	4,853,000
Fair Value	5,107,000
Emerging markets trading debt mutual fund
Investments
Amortized Cost	3,282,000	3,202,000
Fair Value	2,988,000	2,858,000
Emerging markets trading debt securities
Investments
Amortized Cost	605,000	1,300,000
Fair Value	621,000	1,336,000
Other trading investments
Investments
Amortized Cost	505,000	841,000
Fair Value	$ 557,000	$ 918,000