Note 5 - Financial Instruments Measured at Fair Value - Effective Maturities of Short-term Investments (Details) - USD ($) $ in Thousands	Sep. 28, 2019	Dec. 29, 2018
Due after one year through three years, amortized cost	$ 573	$ 560
Due after one year through three years, fair value	$ 573	$ 560

Amount after adjustments of available-for-sale debt securities at cost, maturing in the second fiscal year through the third fiscal year following the latest fiscal year. Adjustments include, but are not limited to, accretion, amortization, collection of cash, previous other-than-temporary impairments (OTTI) recognized in earnings less cumulative-effect adjustments, and fair value hedge accounting adjustments. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

No definition available.

Name:	cohu_AvailableforsaleSecuritiesDebtMaturitiesYearTwoThroughThreeAmortizedCostBasis
Namespace Prefix:	cohu_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant