Segment and Geographic Information Segment and Geographic Information - Summarized Financial Information (Details) - USD ($)	3 Months Ended								12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Segment and geographic information
Assets	$ 1,318,095,000				$ 1,222,923,000				$ 1,318,095,000	$ 1,222,923,000
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property									(700,000)	1,200,000	$ (400,000)
Asset Impairment Charges									1,000,000	184,000	10,300,000
Restructuring charges									8,779,000	14,843,000	5,404,000
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down									1,660,000	(992,000)	10,671,000
Net sales (1)	355,794,000	$ 350,175,000	$ 349,233,000	$ 327,616,000	318,091,000	$ 306,576,000	$ 301,568,000	$ 273,946,000	1,382,818,000	1,200,181,000	757,338,000
Operating income (loss)	54,057,000	$ 61,616,000	$ 50,000,000	$ 35,176,000	21,646,000	$ 32,002,000	$ 31,597,000	$ 27,607,000	200,849,000	112,852,000	34,968,000
Depreciation and amortization (1)									52,628,000	50,661,000	38,151,000
Additions to Other Assets, Amount									146,056,000	65,004,000	25,617,000
Solid Capacitors
Segment and geographic information
Assets	794,402,000				704,851,000				794,402,000	704,851,000
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property										(600,000)
Asset Impairment Charges											2,100,000
Restructuring charges									4,922,000	983,000	1,333,000
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down									235,000	689,000	2,303,000
Operating income (loss)									348,150,000	234,473,000	147,662,000
Depreciation and amortization (1)									28,795,000	27,329,000	20,824,000
Additions to Other Assets, Amount									80,700,000	31,249,000	12,300,000
Film and Electrolytic
Segment and geographic information
Assets	219,711,000				240,968,000				219,711,000	240,968,000
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property										(3,600,000)
Asset Impairment Charges											8,200,000
Restructuring charges									2,717,000	5,788,000	3,738,000
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down									(93,000)	(3,356,000)	8,339,000
Operating income (loss)									8,183,000	3,622,000	(9,028,000)
Depreciation and amortization (1)									9,763,000	10,918,000	11,766,000
Additions to Other Assets, Amount									16,000,000	12,651,000	7,955,000
Electro-magnetic, Sensors & Actuators [Member]
Segment and geographic information
Assets	234,419,000				254,193,000				234,419,000	254,193,000
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property										(1,300,000)
Restructuring charges									452,000	3,343,000	0
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down									0	1,272,000	0
Operating income (loss)									22,546,000	15,694,000	0
Depreciation and amortization (1)									5,226,000	4,407,000	0
Additions to Other Assets, Amount									13,400,000	8,481,000	0
Corporate
Segment and geographic information
Assets	$ 69,563,000				$ 22,911,000				69,563,000	22,911,000
Restructuring charges									688,000	4,729,000	333,000
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down									1,518,000	403,000	29,000
Operating income (loss)									(178,030,000)	(140,937,000)	(103,666,000)
Depreciation and amortization (1)									8,844,000	8,007,000	5,561,000
Additions to Other Assets, Amount									$ 35,956,000	$ 12,623,000	$ 5,362,000