Cash, cash equivalents and short-term investments (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 30, 2016
Fair Value Measurements [Line Items]
Amortized Cost	$ 155,086	$ 173,967
Unrealized Gains	2	0
Unrealized Losses	(185)	(245)
Estimated Fair Value	154,903	173,722
U.S. agency securities
Fair Value Measurements [Line Items]
Amortized Cost	68,000	59,000
Unrealized Gains	2	0
Unrealized Losses	(185)	(245)
Estimated Fair Value	67,817	58,755
Cash
Fair Value Measurements [Line Items]
Amortized Cost	73,948	93,049
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	73,948	93,049
Money market securities
Fair Value Measurements [Line Items]
Amortized Cost	13,138	21,918
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	13,138	21,918
Total cash equivalents
Fair Value Measurements [Line Items]
Amortized Cost	13,138	21,918
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	13,138	21,918
Total cash and cash equivalents
Fair Value Measurements [Line Items]
Amortized Cost	87,086	114,967
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	87,086	114,967
Total short-term investments
Fair Value Measurements [Line Items]
Amortized Cost	68,000	59,000
Unrealized Gains	2	0
Unrealized Losses	(185)	(245)
Estimated Fair Value	$ 67,817	$ 58,755