Cash, cash equivalents and short-term investments (Detail) - USD ($) $ in Thousands	Sep. 29, 2017	Dec. 30, 2016
Fair Value Measurements [Line Items]
Amortized Cost	$ 165,806	$ 173,967
Unrealized Gains	0	0
Unrealized Losses	(215)	(245)
Estimated Fair Value	165,591	173,722
U.S. agency securities
Fair Value Measurements [Line Items]
Amortized Cost	75,997	59,000
Unrealized Gains	0	0
Unrealized Losses	(215)	(245)
Estimated Fair Value	75,782	58,755
Cash
Fair Value Measurements [Line Items]
Amortized Cost	84,544	93,049
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	84,544	93,049
Money market securities
Fair Value Measurements [Line Items]
Amortized Cost	5,265	21,918
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	5,265	21,918
Total cash equivalents
Fair Value Measurements [Line Items]
Amortized Cost	5,265	21,918
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	5,265	21,918
Total cash and cash equivalents
Fair Value Measurements [Line Items]
Amortized Cost	89,809	114,967
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	89,809	114,967
Total short-term investments
Fair Value Measurements [Line Items]
Amortized Cost	75,997	59,000
Unrealized Gains	0	0
Unrealized Losses	(215)	(245)
Estimated Fair Value	$ 75,782	$ 58,755