Note 1 - Summary of Significant Accounting Policies - Cumulative-effect Adjustments (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended			9 Months Ended
		Sep. 29, 2018		Sep. 30, 2017	Sep. 29, 2018		Sep. 30, 2017	Dec. 31, 2017	Dec. 30, 2017
Deferred profit		$ 1,894	[1]		$ 1,894	[1]		$ 5,350	$ 6,608
Income taxes payable		6,072			6,072			2,360	2,159	[2]
Retained earnings		171,110	[1]		171,110	[1]		151,783	$ 150,726	[2]
Net sales		86,164		$ 93,651	281,131		$ 268,614
Income tax provision		2,302		1,837	6,897		4,273
Net income		$ 4,803		$ 8,755	$ 24,573		$ 25,948
Basic: (in dollars per share)		$ 0.17		$ 0.31	$ 0.85		$ 0.94
Diluted: (in dollars per share)		$ 0.16		$ 0.30	$ 0.83		$ 0.91
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Deferred profit	[1]	$ 5,396			$ 5,396
Retained earnings	[1]	168,339			168,339
Net sales		86,820			279,038
Income tax provision		2,421			6,517
Net income		$ 5,340			$ 22,860
Basic: (in dollars per share)		$ 0.18			$ 0.79
Diluted: (in dollars per share)		$ 0.18			$ 0.77
Accounting Standards Update 2014-09 [Member]
Deferred profit								(1,258)
Income taxes payable								201
Retained earnings								$ 1,057
Accounting Standards Update 2014-09 [Member] | Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Deferred profit	[1]	$ (3,502)			$ (3,502)
Retained earnings	[1]	2,771			2,771
Net sales		(656)			2,093
Income tax provision		(119)			380
Net income		$ (537)			$ 1,713
Basic: (in dollars per share)		$ (0.01)			$ 0.06
Diluted: (in dollars per share)		$ (0.02)			$ 0.06

[1] Balance sheet line items include the cumulative-effect adjustment recorded on December 31, 2017.

[2] Derived from December 30, 2017 audited financial statements