Revenue Recognition - Disaggregation of Revenue (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 28,416
Energy Storage (formerly Ultracapacitors)
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	23,002
High-Voltage Capacitors
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	5,414
Americas [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	5,818
Asia Pacific [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	9,827
Europe [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 12,771