Revenue - Revenue from External Customers by Geographic Areas (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Total	$ 115,787	$ 94,413	$ 94,413
North America
Disaggregation of Revenue [Line Items]
Total	97,679	79,947
Pacific Rim
Disaggregation of Revenue [Line Items]
Total	12,625	10,339
Europe
Disaggregation of Revenue [Line Items]
Total	3,377	3,315
Other
Disaggregation of Revenue [Line Items]
Total	2,106	812
Industrial
Disaggregation of Revenue [Line Items]
Total	50,587	45,719
Energy
Disaggregation of Revenue [Line Items]
Total	45,648	38,567
Transportation
Disaggregation of Revenue [Line Items]
Total	$ 19,552	$ 10,127