Segment and Geographic Information Segment and Geographic Information - Summarized Financial Information (Details) - USD ($)	3 Months Ended								9 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Segment and geographic information
Assets	$ 1,218,269,000				$ 734,325,000					$ 1,218,269,000	$ 734,325,000
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property										1,200,000
Asset Impairment Charges										200,000	10,279,000
Restructuring charges										14,843,000	5,404,000	$ 4,178,000
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
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
Additions to Other Assets, Amount										65,004,000	25,617,000	20,469,000
Solid Capacitors
Segment and geographic information
Assets	704,851,000				439,281,000					704,851,000	439,281,000
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property										(600,000)
Asset Impairment Charges									$ 10,300,000		2,100,000
Restructuring charges										983,000	1,333,000	1,916,000
Net sales										771,240,000	575,110,000	556,303,000
Operating income (loss)										234,473,000	147,662,000	130,574,000
Depreciation and amortization										27,329,000	20,824,000	21,318,000
Additions to Other Assets, Amount										31,249,000	12,300,000	10,098,000
Film and Electrolytic
Segment and geographic information
Assets	236,314,000				204,407,000					236,314,000	204,407,000
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										5,788,000	3,738,000	1,714,000
Net sales										201,722,000	182,681,000	178,520,000
Operating income (loss)										4,073,000	(8,200,000)	138,000
Depreciation and amortization										10,012,000	10,953,000	11,984,000
Additions to Other Assets, Amount										12,651,000	7,955,000	5,902,000
Electro-magnetic, Sensors & Actuators [Member]
Segment and geographic information
Assets	254,193,000				0					254,193,000	0
Gain (Loss) on Disposition of Property Plant Equipment, Excluding Oil and Gas Property and Timber Property										(1,300,000)
Restructuring charges										3,343,000	0	0
Net sales										226,964,000	0	0
Operating income (loss)										15,694,000	0	0
Depreciation and amortization										4,407,000	0	0
Additions to Other Assets, Amount										8,481,000	0	0
Corporate
Segment and geographic information
Assets	$ 22,911,000				$ 90,637,000					22,911,000	90,637,000
Restructuring charges										4,729,000	333,000	548,000
Operating income (loss)										(140,937,000)	(103,666,000)	(96,879,000)
Depreciation and amortization										8,007,000	5,561,000	5,714,000
Additions to Other Assets, Amount										$ 12,623,000	$ 5,362,000	$ 4,469,000