Cash, cash equivalents and short-term investments (Detail) - USD ($) $ in Thousands	Dec. 30, 2016	Jan. 01, 2016
Fair Value Measurements [Line Items]
Amortized Cost	$ 173,967	$ 171,699
Unrealized Gains	0	1
Unrealized Losses	(245)	(107)
Estimated Fair Value	173,722	171,593
U.S. agency securities
Fair Value Measurements [Line Items]
Amortized Cost	59,000	41,946
Unrealized Gains	0	1
Unrealized Losses	(245)	(106)
Estimated Fair Value	58,755	41,841
Cash
Fair Value Measurements [Line Items]
Amortized Cost	93,049	115,221
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	93,049	115,221
Money market securities
Fair Value Measurements [Line Items]
Amortized Cost	21,918	10,530
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	21,918	10,530
Total cash equivalents
Fair Value Measurements [Line Items]
Amortized Cost	21,918	10,530
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	21,918	10,530
Total cash and cash equivalents
Fair Value Measurements [Line Items]
Amortized Cost	114,967	125,751
Unrealized Gains	0	0
Unrealized Losses	0	0
Estimated Fair Value	114,967	125,751
State and municipal bonds
Fair Value Measurements [Line Items]
Amortized Cost		4,002
Unrealized Gains		0
Unrealized Losses		(1)
Estimated Fair Value		4,001
Total short-term investments
Fair Value Measurements [Line Items]
Amortized Cost	59,000	45,948
Unrealized Gains	0	1
Unrealized Losses	(245)	(107)
Estimated Fair Value	$ 58,755	$ 45,842