Investments Summary (Details) - USD ($) $ in Thousands	Apr. 01, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Total, amortized cost basis	$ 178,863	$ 183,438
Cash equivalents and available-for-sale investments, gains	13	9
Cash equivalents and available-for-sale securities, losses	350	332
Total, fair value	178,526	183,115
Gross Amortized Cost and Estimated Fair Value of Investments
Due in one year or less, gross amortized cost	122,738	112,164
Due in one year or less, fair value	122,577	112,058
Due in one year through five years, gross amortized cost	56,125	71,274
Due after one year through five years, fair value	55,949	71,057
Total, amortized cost basis	178,863	183,438
Total, fair value	178,526	183,115
Commercial papers
Schedule of Available-for-sale Securities [Line Items]
Total, amortized cost basis	31,702	23,744
Cash equivalents and available-for-sale investments, gains	2	0
Cash equivalents and available-for-sale securities, losses	25	60
Total, fair value	31,679	23,684
Gross Amortized Cost and Estimated Fair Value of Investments
Total, amortized cost basis	31,702	23,744
Total, fair value	31,679	23,684
Money market funds
Schedule of Available-for-sale Securities [Line Items]
Total, amortized cost basis	2,189	2,266
Cash equivalents and available-for-sale investments, gains	0	0
Cash equivalents and available-for-sale securities, losses	0	0
Total, fair value	2,189	2,266
Gross Amortized Cost and Estimated Fair Value of Investments
Total, amortized cost basis	2,189	2,266
Total, fair value	2,189	2,266
U.S. treasury bills and notes
Schedule of Available-for-sale Securities [Line Items]
Total, amortized cost basis	10,485	29,467
Cash equivalents and available-for-sale investments, gains	0	1
Cash equivalents and available-for-sale securities, losses	8	7
Total, fair value	10,477	29,461
Gross Amortized Cost and Estimated Fair Value of Investments
Total, amortized cost basis	10,485	29,467
Total, fair value	10,477	29,461
Securities and obligations of U.S. government agencies
Schedule of Available-for-sale Securities [Line Items]
Total, amortized cost basis	134,487	127,961
Cash equivalents and available-for-sale investments, gains	11	8
Cash equivalents and available-for-sale securities, losses	317	265
Total, fair value	134,181	127,704
Gross Amortized Cost and Estimated Fair Value of Investments
Total, amortized cost basis	134,487	127,961
Total, fair value	134,181	127,704
Cash Equivalents
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	24,129	2,266
Gross Amortized Cost and Estimated Fair Value of Investments
Total, fair value	24,129	2,266
Short-term Investments
Schedule of Available-for-sale Securities [Line Items]
Total, fair value	154,397	180,849
Gross Amortized Cost and Estimated Fair Value of Investments
Total, fair value	$ 154,397	$ 180,849