Shareholders' Equity - Changes in Shareholders' Equity (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 29, 2018	Sep. 30, 2017	Sep. 29, 2018	Sep. 30, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance			$ 927,556
Net income	$ 53,546	$ 42,808	131,900	$ 130,338
Other comprehensive loss	(7,947)	$ (239)	(24,168)	$ 1,236
Stock-based compensation			23,153
Withheld shares on restricted share units for withholding taxes			(7,252)
Stock options exercised			25,171
Issuance of common stock			472,301
Cash dividends paid			(29,258)
Non-controlling interest			(7)
Balance	1,519,396		1,519,396
Littelfuse, Inc. Shareholders’ Equity
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance			927,419
Net income			131,900
Other comprehensive loss			(24,168)
Stock-based compensation			23,153
Withheld shares on restricted share units for withholding taxes			(7,252)
Stock options exercised			25,171
Issuance of common stock			472,301
Cash dividends paid			(29,258)
Balance	1,519,266		1,519,266
Non- controlling Interest
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Balance			137
Non-controlling interest			(7)
Balance	$ 130		$ 130