Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ 33,727	$ 91,607
Energy Storage
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	26,535
High-Voltage Capacitors
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	7,192
Americas [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	7,067	16,970
Asia Pacific [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	14,385	38,929
Europe [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ 12,275	35,708
Ultracapacitors [Member] | Energy Storage
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue		72,242
High-Voltage Capacitors | High-Voltage Capacitors
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue		$ 19,365