Restructuring and Related Activities (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Apr. 01, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Restructuring Reserve [Roll Forward]
Restructuring and severance costs	$ 0	$ 1,469
MOSFETs Enhanced Competitiveness Program [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Reserve, Current	1,980
Restructuring Reserve [Roll Forward]
Restructuring and severance costs		420	$ 3,204	$ 9,744	$ 5,367	$ 6,025	$ 2,328
Cash paid	(283)		(7,173)	(15,686)	(426)	(856)	(267)
Foreign currency translation				2	1
Balance at end of period	1,980		2,263	6,232	12,172	7,230	$ 2,061
Balance at beginning of period	$ 2,263	6,232	6,232	12,172	7,230	$ 2,061
Term of restructuring program	2 years
Global Cost Reduction Program [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring Reserve, Current	$ 4,222
Restructuring Reserve [Roll Forward]
Restructuring and severance costs		1,049	8,069	9,918	13,753
Cash paid	(1,027)		(7,168)	(16,237)	(986)
Foreign currency translation	148		500	(34)	(150)
Balance at end of period	6,786		7,665	6,264	$ 12,617
Balance at beginning of period	$ 7,665	6,264	$ 6,264	$ 12,617
Global Cost Reduction Program [Member] | Resistors And Inductors Segment [Member]
Restructuring Reserve [Roll Forward]
Restructuring and severance costs		851
Global Cost Reduction Program [Member] | Capacitors Segment [Member]
Restructuring Reserve [Roll Forward]
Restructuring and severance costs		161
Global Cost Reduction Program [Member] | Unallocated Amount to Segment [Member]
Restructuring Reserve [Roll Forward]
Restructuring and severance costs		$ 37