Note 1 - Summary of Significant Accounting Policies - Cumulative-effect Adjustments (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2018	Mar. 25, 2017	Dec. 31, 2017	Dec. 30, 2017
Deferred profit	$ 2,914	$ 2,914		$ 5,350	$ 6,608
Income taxes payable	1,546	1,546		2,360	2,159	[1]
Retained earnings	158,124	158,124		151,783	150,726	[1]
Net sales	95,150	95,150	$ 81,097
Income tax provision	2,127	2,127	1,358
Net income	$ 8,122	$ 8,122	$ 6,763
Basic: (in dollars per share)	$ 0.28	$ 0.28	$ 0.25
Diluted: (in dollars per share)	$ 0.28	$ 0.28	$ 0.24
Deferred income taxes	$ 3,812	$ 3,812			$ 2,921	[1]
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Deferred profit	6,664	6,664
Income taxes payable	984	984
Retained earnings	154,819	154,819
Net sales	92,662
Income tax provision	1,883
Net income	$ 5,878
Basic: (in dollars per share)	$ 0.21
Diluted: (in dollars per share)	$ 0.20
Deferred income taxes	$ 3,929	3,929
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	(3,750)	(3,750)
Income taxes payable	562	562
Retained earnings	3,305	3,305
Net sales	2,488
Income tax provision	244
Net income	$ 2,244
Basic: (in dollars per share)	$ 0.07
Diluted: (in dollars per share)	$ 0.08
Deferred income taxes	$ (117)	$ (117)

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