Property and Equipment (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment at cost		$ 3,375	$ 3,411
Less: accumulated depreciation		(2,765)	(2,314)
Property and equipment, net		610	1,097
Depreciation		522	681	$ 805
Impairment loss on equipment and software			200	900
Gain on sale		$ (2)	(203)	$ (38)
Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life		2 years
Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life		15 years
Equipment
Property, Plant and Equipment [Line Items]
Property and equipment at cost		$ 1,511	$ 1,557
Net proceeds	$ 200
Gain on sale	$ 15
Equipment | Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life			3 years	3 years
Equipment | Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life			15 years	15 years
Tooling
Property, Plant and Equipment [Line Items]
Property and equipment at cost		371	$ 371
Tooling | Minimum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life			2 years	2 years
Tooling | Maximum
Property, Plant and Equipment [Line Items]
Property and equipment, useful life			5 years	5 years
Vehicles
Property, Plant and Equipment [Line Items]
Property and equipment at cost		47	$ 47
Property and equipment, useful life			5 years	5 years
Furniture and fixtures
Property, Plant and Equipment [Line Items]
Property and equipment at cost		137	$ 137
Property and equipment, useful life			5 years	5 years
Computer software
Property, Plant and Equipment [Line Items]
Property and equipment at cost		1,043	$ 1,043
Property and equipment, useful life			3 years	3 years
Leasehold improvements
Property, Plant and Equipment [Line Items]
Property and equipment at cost		211	$ 201
Construction in progress
Property, Plant and Equipment [Line Items]
Property and equipment at cost		$ 55	$ 55