Investments Summary (Details) - USD ($) $ in Thousands	Apr. 02, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Total, amortized cost basis	$ 194,576	$ 204,342
Cash equivalents and available-for-sale investments, gains	39	4
Cash equivalents and available-for-sale securities, losses	94	381
Total, fair value	194,521	203,965
Gross Amortized Cost and Estimated Fair Value of Investments
Due in one year or less, gross amortized cost	150,071	155,342
Due in one year or less, fair value	150,030	155,177
Due in one year through five years, gross amortized cost	44,505	49,000
Due after one year through five years, fair value	44,491	48,788
Total, amortized cost basis	194,576	204,342
Total, fair value	194,521	203,965
Commercial papers
Schedule of Available-for-sale Securities [Line Items]
Total, amortized cost basis	7,995	9,996
Cash equivalents and available-for-sale investments, gains	0	0
Cash equivalents and available-for-sale securities, losses	0	0
Total, fair value	7,995	9,996
Gross Amortized Cost and Estimated Fair Value of Investments
Total, amortized cost basis	7,995	9,996
Total, fair value	7,995	9,996
Money market funds
Schedule of Available-for-sale Securities [Line Items]
Total, amortized cost basis	11,364	9,074
Cash equivalents and available-for-sale investments, gains	0	0
Cash equivalents and available-for-sale securities, losses	0	0
Total, fair value	11,364	9,074
Gross Amortized Cost and Estimated Fair Value of Investments
Total, amortized cost basis	11,364	9,074
Total, fair value	11,364	9,074
U.S. treasury bills and notes
Schedule of Available-for-sale Securities [Line Items]
Total, amortized cost basis	51,958	56,929
Cash equivalents and available-for-sale investments, gains	16	1
Cash equivalents and available-for-sale securities, losses	1	58
Total, fair value	51,973	56,872
Gross Amortized Cost and Estimated Fair Value of Investments
Total, amortized cost basis	51,958	56,929
Total, fair value	51,973	56,872
Securities and obligations of U.S. government agencies
Schedule of Available-for-sale Securities [Line Items]
Total, amortized cost basis	123,259	128,343
Cash equivalents and available-for-sale investments, gains	23	3
Cash equivalents and available-for-sale securities, losses	93	323
Total, fair value	123,189	128,023
Gross Amortized Cost and Estimated Fair Value of Investments
Total, amortized cost basis	123,259	128,343
Total, fair value	123,189	128,023
Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	27,218	15,072
Gross Amortized Cost and Estimated Fair Value of Investments
Total, fair value	27,218	15,072
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	167,303	188,893
Gross Amortized Cost and Estimated Fair Value of Investments
Total, fair value	$ 167,303	$ 188,893