Effect of Changes in Balance Sheet Due to Adoption of New Revenue Standard (Detail) - USD ($) $ in Thousands	Sep. 29, 2018	Dec. 31, 2017	Dec. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other accrued liabilities	$ 10,995		$ 7,688
Accumulated deficit	(74,917)		$ (66,881)
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other accrued liabilities	9,697	$ 9,322
Accumulated deficit	(73,619)	(68,515)
Accounting Standards Update 2014-09 | Difference Between Revenue Guidance in Effect Before and After Topic 606
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other accrued liabilities	1,298	1,634
Accumulated deficit	$ (1,298)	$ (1,634)