Investments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 29, 2014	Dec. 31, 2013
Investments
Total available-for-sale and trading short term investments, amortized cost	$ 390,973	$ 290,042
Total available-for-sale and trading short term investments, fair value	392,222	290,649
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	5,378
Due after one year through three years	44,186
Due after three years	48,331
Total fixed rate securities	97,895
Available-for-sale securities
Investments
Amortized Cost	334,485	224,314
Fair Value	334,818	224,076
Money market funds
Investments
Amortized Cost	216,623	88,430
Fair Value	216,623	88,430
Money market funds | Denominated in Euros
Investments
Fair Value	6,329
Money market funds | Denominated in Canadian dollars
Investments
Fair Value	3,359
Corporate bonds
Investments
Amortized Cost	49,044	69,591
Fair Value	49,482	70,258
U.S. Government agency securities
Investments
Amortized Cost	47,620	27,299
Fair Value	47,554	27,147
Asset backed debt securities
Investments
Amortized Cost	11,949	8,446
Fair Value	11,938	8,477
Collateralized mortgage obligations
Investments
Amortized Cost	6,441	7,597
Fair Value	6,451	7,600
U.S. Treasury securities
Investments
Amortized Cost	2,808	5,258
Fair Value	2,770	5,223
Emerging markets debt mutual fund
Investments
Amortized Cost		17,693
Fair Value		16,941
High yield trading debt securities
Investments
Amortized Cost	50,696	49,352
Fair Value	51,923	50,428
Emerging markets trading debt mutual fund
Investments
Amortized Cost	3,238	3,202
Fair Value	2,856	2,858
Money market funds held in trading accounts
Investments
Amortized Cost	1,287	11,033
Fair Value	1,287	11,033
Emerging markets trading debt securities
Investments
Amortized Cost	712	1,300
Fair Value	729	1,336
Other trading investments
Investments
Amortized Cost	555	841
Fair Value	$ 609	$ 918