Cash, cash equivalents and short-term investments (Detail) - USD ($) $ in Thousands	Sep. 27, 2019	Dec. 28, 2018
Fair Value Measurements [Line Items]
Amortized Cost	$ 209,610	$ 208,693
Gross Unrealized Gains	361	91
Gross Unrealized Losses	(3)	(230)
Estimated Fair Value	209,968	208,554
Cash
Fair Value Measurements [Line Items]
Amortized Cost	110,052	120,846
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	110,052	120,846
Money market securities
Fair Value Measurements [Line Items]
Amortized Cost	22,791	6,213
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	22,791	6,213
Total cash equivalents
Fair Value Measurements [Line Items]
Amortized Cost	22,791	6,213
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	22,791	6,213
Total cash and cash equivalents
Fair Value Measurements [Line Items]
Amortized Cost	132,843	127,059
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	132,843	127,059
U.S. Treasury and agency securities
Fair Value Measurements [Line Items]
Amortized Cost	76,767	81,634
Gross Unrealized Gains	361	91
Gross Unrealized Losses	(3)	(230)
Estimated Fair Value	77,125	81,495
Total short-term investments
Fair Value Measurements [Line Items]
Amortized Cost	76,767	81,634
Gross Unrealized Gains	361	91
Gross Unrealized Losses	(3)	(230)
Estimated Fair Value	$ 77,125	$ 81,495