Cash, cash equivalents and short-term investments (Detail) - USD ($) $ in Thousands	Mar. 29, 2019	Dec. 29, 2017
Fair Value Measurements [Line Items]
Amortized Cost	$ 171,693	$ 208,693
Gross Unrealized Gains	174	91
Gross Unrealized Losses	(107)	(230)
Estimated Fair Value	171,760	208,554
U.S. Treasury and agency securities
Fair Value Measurements [Line Items]
Amortized Cost	92,613	81,634
Gross Unrealized Gains	174	91
Gross Unrealized Losses	(107)	(230)
Estimated Fair Value	92,680	81,495
Cash
Fair Value Measurements [Line Items]
Amortized Cost	73,310	120,846
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	73,310	120,846
Money market securities
Fair Value Measurements [Line Items]
Amortized Cost	770	6,213
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	770	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	5,770	6,213
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	5,770	6,213
Total cash and cash equivalents
Fair Value Measurements [Line Items]
Amortized Cost	79,080	127,059
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	79,080	127,059
Total short-term investments
Fair Value Measurements [Line Items]
Amortized Cost	92,613	81,634
Gross Unrealized Gains	174	91
Gross Unrealized Losses	(107)	(230)
Estimated Fair Value	$ 92,680	$ 81,495