Note 17 - Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 26, 2020	[1]	Sep. 26, 2020	[1]	Jun. 27, 2020	[1]	Mar. 28, 2020	[1]	Dec. 28, 2019	[1]	Sep. 28, 2019	[1]	Jun. 29, 2019	[1]	Mar. 30, 2019	[1]	Dec. 26, 2020	Dec. 28, 2019	Dec. 29, 2018
Net sales	$ 202,355		$ 150,647		$ 144,084		$ 138,921		$ 142,011		$ 143,498		$ 150,011		$ 147,809		$ 636,007	$ 583,329	$ 451,768
Cost of sales:	111,114	[2]	87,147	[2]	83,127	[2]	82,837	[2]	87,936	[2]	84,565	[2]	87,605	[2]	93,394	[2]	364,225	353,500
Income (loss) from continuing operations	14,861		(6,646)		(4,740)		(17,318)		(16,281)		(10,480)		(19,383)		(22,851)		(13,843)	(68,995)	(32,543)
Net income (loss)	14,861		(6,646)		(4,740)		(17,276)		(17,320)		(10,326)	[2]	(19,359)		(22,687)		(13,801)	(69,692)	(32,424)
Net income (loss) attributable to Cohu	$ 14,861		$ (6,646)		$ (4,740)		$ (17,276)		$ (17,266)		$ (10,468)		$ (19,323)		$ (22,643)		$ (13,801)	$ (69,700)	$ (32,181)
Income (loss) from continuing operations (in dollars per share)	$ 0.35	[3]	$ (0.16)	[3]	$ (0.11)	[3]	$ (0.42)	[3]	$ (0.39)	[3]	$ (0.25)	[3]	$ (0.47)	[3]	$ (0.56)	[3]	$ (0.33)	$ (1.68)	$ (1.02)
Net income (loss) (in dollars per share)	0.35	[3]	(0.16)	[3]	(0.11)	[3]	(0.42)	[3]	(0.42)	[3]	(0.25)	[3]	(0.47)	[3]	(0.55)	[3]	(0.33)	(1.69)	(1.01)
Income (loss) from continuing operations (in dollars per share)	0.34	[3]	(0.16)	[3]	(0.11)	[3]	(0.42)	[3]	(0.39)	[3]	(0.25)	[3]	(0.47)	[3]	(0.56)	[3]	(0.33)	(1.68)	(1.02)
Net income (loss) (in dollars per share)	$ 0.34	[3]	$ (0.16)	[3]	$ (0.11)	[3]	$ (0.42)	[3]	$ (0.42)	[3]	$ (0.25)	[3]	$ (0.47)	[3]	$ (0.55)	[3]	$ (0.33)	$ (1.69)	$ (1.01)

[1] All quarters presented above were comprised of 13 weeks.

[2] Cost of sales is shown exclusive of the amortization of purchased intangible assets.

[3] The sum of the four quarters may not agree to the year total due to rounding or losses within a quarter and the inclusion or exclusion of common stock equivalents.