Note 2 - Acquisition - Pro Forma Information Adjustments (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jul. 01, 2017	Jun. 30, 2018	Jul. 01, 2017
Net income			$ 37,442	$ 106,247	$ 55,043
Adjustment to Amortization [Member]
Net income	[1]	$ 5,185	(6,305)		(12,601)
Adjustment to Depreciation [Member]
Net income			139		278
Acquisition-related Costs [Member]
Net income	[2]	9,976			(9,976)
Amortization of Inventory Step Up [Member]
Net income	[3]	36,927	(14,771)		(36,927)
Adjustment to Stock Compensation [Member]
Net income	[4]	4,689	(767)		(5,780)
Adjustment to Interest Expense [Member]
Net income	[5]		(2,582)		(5,164)
Adjustment to Income Tax [Member]
Net income		$ (13,329)	$ 7,596		$ 22,896

[1] The amortization adjustment for the six months ended June 30, 2018 primarily reflects the reduction of amortization expense in the period related to the Order backlog intangible asset. The Order backlog has a useful life of twelve months and will be fully amortized in the fiscal 2017 pro forma results. The amortization adjustment for the three and six months ended July 1, 2017 reflects incremental amortization resulting for the measurement of intangibles at their fair values.

[2] The transaction cost adjustments reflect the reversal of certain bank and attorney fees from the six months ended June 30, 2018 and recognition of those fees during the six months ended July 1, 2017.

[3] The amortization of inventory step-up adjustment reflects the reversal of the amount recognized during the six months ended June 30, 2018 and the recognition of the full amortization during the six months end July 1, 2017. The inventory step-up was amortized over five months as the inventory is sold.

[4] The stock compensation adjustment reflects the reversal of the portion of stock compensation for IXYS stock options that were converted to Littelfuse stock options and expensed immediately during the six months ended June 30, 2018. The adjustment for the six months ended July 1, 2017 reflect the incremental stock compensation for the converted stock options.

[5] The interest expense adjustment reflects incremental interest expense related to the financing of the transaction.