Note 9 - Income Taxes (Details Textual) - USD ($) $ in Millions	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2018	Dec. 30, 2017	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017	Dec. 29, 2018	Dec. 30, 2017	Dec. 31, 2017
Effective Income Tax Rate Reconciliation, Percent, Total	19.10%		15.20%	19.20%	16.50%
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
Other Long-term Liabilities [Member]
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability, Toll Charge, Noncurrent	$ 38.5
Accrued Income Taxes [Member]
Income Tax Expense (Benefit), Continuing Operations, Adjustment of Deferred Tax (Asset) Liability, Toll Charge, Current	$ 3.4
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%