Segment and Geographic Information - Net Sales by Country (Details) - USD ($) $ in Thousands	3 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	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2016
Net sales and property, plant and equipment, goodwill, intangible assets
Net sales	$ 318,047	$ 306,408	$ 301,471	$ 274,000	$ 197,519	$ 188,029	$ 187,308	$ 184,935	$ 1,199,926	$ 757,791	$ 734,823
Property, plant and equipment, net	405,316				209,311				405,316	209,311
Solid Capacitors
Net sales and property, plant and equipment, goodwill, intangible assets
Net sales									771,240	575,110	556,303
Film and Electrolytic
Net sales and property, plant and equipment, goodwill, intangible assets
Net sales									$ 201,722	182,681	178,520
Net sales | Sales risk
Net sales and property, plant and equipment, goodwill, intangible assets
Number of customers									1
Concentration Risk, Geographic									No
Net sales									$ 133,400	104,400	99,300
Net sales | Geographic risk
Net sales and property, plant and equipment, goodwill, intangible assets
Threshold for Disclosure Percentage									3.00%
Property, Plant and Equipment [Member] | Geographic risk
Net sales and property, plant and equipment, goodwill, intangible assets
Threshold for Disclosure Percentage									1.6564%
United States
Net sales and property, plant and equipment, goodwill, intangible assets
Net sales									$ 233,133	198,250	201,878
Property, plant and equipment, net	49,530				46,387				49,530	46,387
Japan
Net sales and property, plant and equipment, goodwill, intangible assets
Net sales									170,282	3,565	5,251
Property, plant and equipment, net	79,855				0				79,855	0
Thailand
Net sales and property, plant and equipment, goodwill, intangible assets
Property, plant and equipment, net	74,100				0				74,100	0
China
Net sales and property, plant and equipment, goodwill, intangible assets
Net sales									163,016	71,223	65,043
Property, plant and equipment, net	36,396				23,780				36,396	23,780
Italy
Net sales and property, plant and equipment, goodwill, intangible assets
Net sales									17,905	15,376	14,391
Property, plant and equipment, net	39,398				36,108				39,398	36,108
Hungary
Net sales and property, plant and equipment, goodwill, intangible assets
Net sales									13,254	18,856	17,766
Mexico
Net sales and property, plant and equipment, goodwill, intangible assets
Net sales									23,915	22,424	23,041
Property, plant and equipment, net	62,503				55,878				62,503	55,878
Other
Net sales and property, plant and equipment, goodwill, intangible assets
Net sales									2,278	4,131	1,661
Property, plant and equipment, net	14,733				7,331				14,733	7,331
Portugal
Net sales and property, plant and equipment, goodwill, intangible assets
Property, plant and equipment, net	29,073				21,637				29,073	21,637
Macedonia
Net sales and property, plant and equipment, goodwill, intangible assets
Property, plant and equipment, net	13,723				12,339				13,723	12,339
Sweden
Net sales and property, plant and equipment, goodwill, intangible assets
Property, plant and equipment, net	6,005				5,851				6,005	5,851
Total Non-United States
Net sales and property, plant and equipment, goodwill, intangible assets
Net sales									966,793	559,541	532,945
Property, plant and equipment, net	$ 355,786				$ 162,924				355,786	162,924
Hong Kong
Net sales and property, plant and equipment, goodwill, intangible assets
Net sales									169,073	121,813	133,117
Germany
Net sales and property, plant and equipment, goodwill, intangible assets
Net sales									105,548	104,755	83,589
Europe
Net sales and property, plant and equipment, goodwill, intangible assets
Net sales									63,993	64,316	57,362
Taiwan
Net sales and property, plant and equipment, goodwill, intangible assets
Net sales									78,728	9,147	8,747
Asia Pacific
Net sales and property, plant and equipment, goodwill, intangible assets
Net sales									36,647	26,878	32,426
United Kingdom
Net sales and property, plant and equipment, goodwill, intangible assets
Net sales									37,038	33,837	28,083
Netherlands
Net sales and property, plant and equipment, goodwill, intangible assets
Net sales									39,684	32,478	31,218
Malaysia
Net sales and property, plant and equipment, goodwill, intangible assets
Net sales									28,165	15,177	14,990
Singapore
Net sales and property, plant and equipment, goodwill, intangible assets
Net sales									$ 17,267	$ 15,565	$ 16,260
Minimum | Net sales | Sales risk
Net sales and property, plant and equipment, goodwill, intangible assets
Concentration risk, percentage									0.00%