Cash, cash equivalents and short-term investments (Detail) - USD ($) $ in Thousands	Jun. 28, 2019	Dec. 28, 2018
Fair Value Measurements [Line Items]
Amortized Cost	$ 194,075	$ 208,693
Gross Unrealized Gains	423	91
Gross Unrealized Losses	(24)	(230)
Estimated Fair Value	194,474	208,554
Cash
Fair Value Measurements [Line Items]
Amortized Cost	95,230	120,846
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	95,230	120,846
Money market securities
Fair Value Measurements [Line Items]
Amortized Cost	4,222	6,213
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	4,222	6,213
U.S. Treasury securities
Fair Value Measurements [Line Items]
Amortized Cost	5,000
Gross Unrealized Gains	0
Gross Unrealized Losses	0
Estimated Fair Value	5,000
Total cash equivalents
Fair Value Measurements [Line Items]
Amortized Cost	9,222	6,213
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	9,222	6,213
Total cash and cash equivalents
Fair Value Measurements [Line Items]
Amortized Cost	104,452	127,059
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	104,452	127,059
U.S. Treasury and agency securities
Fair Value Measurements [Line Items]
Amortized Cost	89,623	81,634
Gross Unrealized Gains	423	91
Gross Unrealized Losses	(24)	(230)
Estimated Fair Value	90,022	81,495
Total short-term investments
Fair Value Measurements [Line Items]
Amortized Cost	89,623	81,634
Gross Unrealized Gains	423	91
Gross Unrealized Losses	(24)	(230)
Estimated Fair Value	$ 90,022	$ 81,495