Segment and Geographic Information - Narrative (Details)		3 Months Ended								12 Months Ended
	Apr. 19, 2017 business_group	Mar. 31, 2018 USD ($) manufacturing_site	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2018 USD ($) manufacturing_site business_group	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)
Segment Reporting [Abstract]
Number of business groups | business_group	2									3
Segment and geographic information
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down										$ (992,000)	$ 10,671,000	$ 375,000
Net sales		$ 318,047,000	$ 306,408,000	$ 301,471,000	$ 274,000,000	$ 197,519,000	$ 188,029,000	$ 187,308,000	$ 184,935,000	1,199,926,000	757,791,000	734,823,000
Write down of long-lived assets												0
Assets		1,218,269,000				734,325,000				1,218,269,000	734,325,000
Operating income (loss)		21,799,000	$ 32,077,000	$ 31,643,000	$ 27,784,000	8,942,000	$ 14,182,000	$ 3,374,000	$ 9,298,000	113,303,000	35,796,000	33,833,000
Depreciation and amortization										49,755,000	37,338,000	39,016,000
Solid Capacitors
Segment and geographic information
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down										689,000	2,303,000	536,000
Net sales										771,240,000	575,110,000	556,303,000
Assets		$ 704,851,000				439,281,000				$ 704,851,000	439,281,000
Number of manufacturing sites | manufacturing_site		10								10
Operating income (loss)										$ 234,473,000	147,662,000	130,574,000
Depreciation and amortization										27,329,000	20,824,000	21,318,000
Film and Electrolytic
Segment and geographic information
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down										(3,356,000)	8,339,000	(270,000)
Net sales										201,722,000	182,681,000	178,520,000
Assets		$ 236,314,000				204,407,000				$ 236,314,000	204,407,000
Number of manufacturing sites | manufacturing_site		10								10
Operating income (loss)										$ 4,073,000	(8,200,000)	138,000
Depreciation and amortization										10,012,000	10,953,000	11,984,000
Electro-magnetic, Sensors & Actuators [Member]
Segment and geographic information
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down										1,272,000	0	0
Net sales										226,964,000	0	0
Assets		$ 254,193,000				0				$ 254,193,000	0
Number of manufacturing sites | manufacturing_site		4								4
Operating income (loss)										$ 15,694,000	0	0
Depreciation and amortization										4,407,000	0	0
Corporate
Segment and geographic information
Disposal Group, Not Discontinued Operation, Loss (Gain) on Write-down										403,000	29,000	109,000
Assets		$ 22,911,000				$ 90,637,000				22,911,000	90,637,000
Operating income (loss)										(140,937,000)	(103,666,000)	(96,879,000)
Depreciation and amortization										8,007,000	5,561,000	5,714,000
Sales Revenue, Goods, Net [Member] | Sales risk
Segment and geographic information
Net sales										$ 133,400,000	$ 104,400,000	$ 99,300,000