Revenue Recognition - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ 29,464	$ 57,880
Energy Storage
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	22,705
High-Voltage Capacitors
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	6,759
Americas [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	4,085	9,903
Asia Pacific [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	14,717	24,544
Europe [Member]
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue	$ 10,662	23,433
Ultracapacitors [Member] | Energy Storage
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue		45,707
High-Voltage Capacitors | High-Voltage Capacitors
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Revenue		$ 12,173