Note 9 - Income Taxes (Details Textual) - USD ($) $ in Millions	3 Months Ended			12 Months Ended
	Mar. 31, 2018	Dec. 30, 2017	Apr. 01, 2017	Dec. 29, 2018	Dec. 30, 2017
Effective Income Tax Rate Reconciliation, Percent, Total	19.30%		18.00%
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent					35.00%
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability		$ 47.0
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability, Toll Charge		49.0
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability, Other Than Toll Charge		$ (2.0)
Deferred Tax Liabilities, Undistributed Foreign Earnings	$ 12.0
Foreign Tax Authority [Member]
Deferred Tax Liabilities, Undistributed Foreign Earnings	11.8
Domestic Tax Authority [Member]
Deferred Tax Liabilities, Undistributed Foreign Earnings	0.2
IXYS Corporation [Member]
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability, Toll Charge	$ 10.0
Scenario, Forecast [Member]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent				21.00%