Description of Business and Summary of Significant Accounting Policies (Revenues by product line and geographic area) (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Revenue from External Customer [Line Items]
Total		$ 30,763	$ 35,816	$ 37,103	$ 26,686	$ 26,400	$ 25,506	$ 34,135	$ 35,203	$ 130,368	[1]	$ 121,244	[1]	$ 167,372	[1]
China
Revenue from External Customer [Line Items]
Total	[1]									44,945		48,191		87,856
United States
Revenue from External Customer [Line Items]
Total	[1]									13,874		12,041		20,836
Germany
Revenue from External Customer [Line Items]
Total	[1]									16,287		12,854		13,972
HUNGARY
Revenue from External Customer [Line Items]
Total	[1]									13,454		11,473		11,630
All other countries
Revenue from External Customer [Line Items]
Total	[1],[2]									41,808		36,685		33,078
Ultracapacitors
Revenue from External Customer [Line Items]
Total										87,709		71,491		114,525
High-voltage capacitors
Revenue from External Customer [Line Items]
Total										42,659		45,177		41,718
Microelectronic products
Revenue from External Customer [Line Items]
Total										$ 0		$ 4,576		$ 11,129

[1] Location is determined by shipment destination.

[2] Revenue from external customers located in countries included in “All other countries” does not individually comprise more than 10% of total revenue for any of the years presented.