Segment Information (Details) $ in Thousands	3 Months Ended					12 Months Ended
	Mar. 31, 2018 USD ($) Segment	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jul. 01, 2017 USD ($)	Apr. 01, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Segment Reporting Information [Line Items]
Number of reportable segments | Segment	5
Product Sales	$ 716,756				$ 604,795
Royalty Revenues	39				6
Total Revenue	716,795	$ 673,462	$ 677,941	$ 643,164	604,801	$ 2,599,368	$ 2,317,328
Gross Margin	205,300	177,376	190,147	173,837	161,749	703,109	573,822
Segment Operating Income	172,178				130,510
Restructuring and severance Costs	0				(1,469)
Operating income	104,062	76,006	95,416	85,005	67,578	324,005	197,058
Unallocated Other Income (Expense)	(12,043)	(8,981)	(9,228)	(9,296)	(17,136)	(44,641)	(102,842)
Income before taxes	92,019	$ 67,025	$ 86,188	$ 75,709	50,442	279,364	94,216
Distributors [Member]
Segment Reporting Information [Line Items]
Total Revenue	404,060				345,703	1,484,276	1,280,060
OEMs [Member]
Segment Reporting Information [Line Items]
Total Revenue	264,050				214,647	931,291	861,322
EMS companies [Member]
Segment Reporting Information [Line Items]
Total Revenue	48,685				44,451	183,801	175,946
Industrial [Member]
Segment Reporting Information [Line Items]
Total Revenue	280,212				216,766	934,631	796,031
Automotive [Member]
Segment Reporting Information [Line Items]
Total Revenue	208,394				175,343	727,220	640,767
Telecommunications [Member]
Segment Reporting Information [Line Items]
Total Revenue	45,924				45,835	190,682	193,456
Computing [Member]
Segment Reporting Information [Line Items]
Total Revenue	47,431				41,070	198,850	172,481
Consumer Products [Member]
Segment Reporting Information [Line Items]
Total Revenue	37,259				29,607	145,243	150,741
Power Supplies [Member]
Segment Reporting Information [Line Items]
Total Revenue	34,243				34,858	160,038	132,555
Military and Aerospace [Member]
Segment Reporting Information [Line Items]
Total Revenue	35,214				33,176	132,898	128,523
Medical [Member]
Segment Reporting Information [Line Items]
Total Revenue	28,118				28,146	109,806	102,774
Asia [Member]
Segment Reporting Information [Line Items]
Total Revenue	285,478				257,058	1,091,107	948,195
Americas [Member]
Segment Reporting Information [Line Items]
Total Revenue	163,935				141,718	605,904	558,590
Europe [Member]
Segment Reporting Information [Line Items]
Total Revenue	267,382				206,025	$ 902,357	$ 810,543
MOSFETS Segment [Member]
Segment Reporting Information [Line Items]
Product Sales	127,494				104,842
Royalty Revenues	12				0
Total Revenue	127,506				104,842
Gross Margin	32,022				20,717
Segment Operating Income	22,558				11,803
Diodes Segment [Member]
Segment Reporting Information [Line Items]
Product Sales	167,017				144,585
Royalty Revenues	0				0
Total Revenue	167,017				144,585
Gross Margin	43,200				38,162
Segment Operating Income	37,931				33,442
Optoelectronic Components Segment [Member]
Segment Reporting Information [Line Items]
Product Sales	71,958				65,262
Royalty Revenues	0				0
Total Revenue	71,958				65,262
Gross Margin	27,233				22,442
Segment Operating Income	22,794				17,117
Resistors And Inductors Segment [Member]
Segment Reporting Information [Line Items]
Product Sales	244,019				200,246
Royalty Revenues	27				6
Total Revenue	244,046				200,252
Gross Margin	78,530				61,215
Segment Operating Income	70,002				53,958
Capacitors Segment [Member]
Segment Reporting Information [Line Items]
Product Sales	106,268				89,860
Royalty Revenues	0				0
Total Revenue	106,268				89,860
Gross Margin	24,315				19,213
Segment Operating Income	18,893				14,190
Unallocated Amount to Segment [Member]
Segment Reporting Information [Line Items]
Segment Operating Income	(68,116)				(61,463)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Segment Operating Income	172,178				130,510
Unallocated Amount to Segment [Member]
Segment Reporting Information [Line Items]
Restructuring and severance Costs	0				(1,469)
Unallocated Other Income (Expense)	$ (12,043)				$ (17,136)