Intangible Assets and Goodwill - Intangible Assets (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 5,199	$ 4,523
Accumulated amortization	(2,838)	(2,687)
Net value	$ 2,361	$ 1,836
Patents
Finite-Lived Intangible Assets [Line Items]
Average amortization period	5 years	5 years
Gross carrying amount	$ 3,971	$ 3,310
Accumulated amortization	(1,610)	(1,474)
Net value	$ 2,361	$ 1,836
Arbor acquired technology
Finite-Lived Intangible Assets [Line Items]
Average amortization period	3 years	3 years
Gross carrying amount	$ 670	$ 670
Accumulated amortization	(670)	(670)
Net value	$ 0	$ 0
Direct nanoparticle deposition technology
Finite-Lived Intangible Assets [Line Items]
Average amortization period	6 years	6 years
Gross carrying amount	$ 558	$ 543
Accumulated amortization	(558)	(543)
Net value	$ 0	$ 0