Impact of New Revenue Standard on Consolidated Balance Sheet (Detail) - USD ($) $ in Thousands	Sep. 29, 2018	Jun. 30, 2018	Dec. 31, 2017	Dec. 30, 2017	Sep. 30, 2017	Jul. 01, 2017	Jan. 01, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other accrued liabilities	$ 10,995			$ 7,688
Total current liabilities	32,839			29,481
Accumulated deficit	(74,917)			(66,881)
Total stockholders' equity	79,342	$ 78,586		$ 82,663	$ 81,418	$ 78,107	$ 73,266
Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other accrued liabilities	9,697		$ 9,322
Total current liabilities	31,541
Accumulated deficit	(73,619)		(68,515)
Total stockholders' equity	80,640
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
Total current liabilities	1,298
Accumulated deficit	(1,298)		$ (1,634)
Total stockholders' equity	$ (1,298)