Investments Summary (Details) - USD ($) $ in Thousands	Jul. 04, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Total, amortized cost basis	$ 228,558	$ 228,319
Cash equivalents and available-for-sale investments, gains	31	5
Cash equivalents and available-for-sale securities, losses	110	126
Total, fair value	228,479	228,198
Gross Amortized Cost and Estimated Fair Value of Investments
Due in one year or less, gross amortized cost	198,558	183,317
Due in one year or less, fair value	198,512	183,234
Due in one year through five years, gross amortized cost	30,000	45,002
Due after one year through five years, fair value	29,967	44,964
Total, amortized cost basis	228,558	228,319
Total, fair value	228,479	228,198
Commercial papers
Schedule of Available-for-sale Securities [Line Items]
Total, amortized cost basis	9,997	9,998
Cash equivalents and available-for-sale investments, gains	0	0
Cash equivalents and available-for-sale securities, losses	0	0
Total, fair value	9,997	9,998
Gross Amortized Cost and Estimated Fair Value of Investments
Total, amortized cost basis	9,997	9,998
Total, fair value	9,997	9,998
Money market funds
Schedule of Available-for-sale Securities [Line Items]
Total, amortized cost basis	14,794	26,456
Cash equivalents and available-for-sale investments, gains	0	0
Cash equivalents and available-for-sale securities, losses	0	0
Total, fair value	14,794	26,456
Gross Amortized Cost and Estimated Fair Value of Investments
Total, amortized cost basis	14,794	26,456
Total, fair value	14,794	26,456
U.S. treasury bills and notes
Schedule of Available-for-sale Securities [Line Items]
Total, amortized cost basis	24,954	3,918
Cash equivalents and available-for-sale investments, gains	3	1
Cash equivalents and available-for-sale securities, losses	6	0
Total, fair value	24,951	3,919
Gross Amortized Cost and Estimated Fair Value of Investments
Total, amortized cost basis	24,954	3,918
Total, fair value	24,951	3,919
Securities and obligations of U.S. government agencies
Schedule of Available-for-sale Securities [Line Items]
Total, amortized cost basis	178,813	187,947
Cash equivalents and available-for-sale investments, gains	28	4
Cash equivalents and available-for-sale securities, losses	104	126
Total, fair value	178,737	187,825
Gross Amortized Cost and Estimated Fair Value of Investments
Total, amortized cost basis	178,813	187,947
Total, fair value	178,737	187,825
Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	18,792	26,456
Gross Amortized Cost and Estimated Fair Value of Investments
Total, fair value	18,792	26,456
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	209,687	201,742
Gross Amortized Cost and Estimated Fair Value of Investments
Total, fair value	$ 209,687	$ 201,742