Goodwill and Other Intangibles - Narrative (Details) - USD ($) $ in Thousands	1 Months Ended	6 Months Ended	12 Months Ended
	Mar. 31, 2018	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Goodwill		$ 29,835	$ 29,835
Accumulated impairment losses		(11,600)	(11,600)
Asset impairment charges		$ 2,200
Asset Purchase Agreement - Alternative Fuel Supply Technology
Finite-Lived Intangible Assets [Line Items]
Guaranteed payment	$ 1,200
Additional payment for future sales milestones	3,300
Intellectual Property | Asset Purchase Agreement - Alternative Fuel Supply Technology
Finite-Lived Intangible Assets [Line Items]
Intangible assets acquired	1,500		$ 1,000	$ 500
Developed technology | Asset Purchase Agreement - Alternative Fuel Supply Technology
Finite-Lived Intangible Assets [Line Items]
Intangible assets acquired	$ 2,700