Cash, cash equivalents and short-term investments (Detail) - USD ($) $ in Thousands	Sep. 28, 2018	Dec. 29, 2017
Fair Value Measurements [Line Items]
Amortized Cost	$ 202,814	$ 196,791
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(464)	(393)
Estimated Fair Value	202,350	196,398
U.S. Treasury and agency securities
Fair Value Measurements [Line Items]
Amortized Cost	81,837	71,997
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(464)	(393)
Estimated Fair Value	81,373	71,604
Cash
Fair Value Measurements [Line Items]
Amortized Cost	120,372	115,052
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	120,372	115,052
Money market securities
Fair Value Measurements [Line Items]
Amortized Cost	605	9,742
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	605	9,742
Total cash equivalents
Fair Value Measurements [Line Items]
Amortized Cost	605	9,742
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	605	9,742
Total cash and cash equivalents
Fair Value Measurements [Line Items]
Amortized Cost	120,977	124,794
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	120,977	124,794
Total short-term investments
Fair Value Measurements [Line Items]
Amortized Cost	81,837	71,997
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(464)	(393)
Estimated Fair Value	$ 81,373	$ 71,604