Intangible Assets - Gross Carrying Amount (Details) - USD ($) $ in Thousands	6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	May 31, 2019
Gross carrying amount and accumulated amortization of acquired identifiable intangible assets
Gross Carrying Amount	$ 8,077		$ 6,246
Accumulated Amortization	(2,679)		(2,356)
Total	5,398		$ 3,890
Payments to Acquire Intangible Assets	1,860	$ 879
EnergyOr
Gross carrying amount and accumulated amortization of acquired identifiable intangible assets
Milestone payments				$ 3,000
Acquired				$ 1,500
American Fuel Cell LLC
Gross carrying amount and accumulated amortization of acquired identifiable intangible assets
Milestone payments	2,900
Payments to Acquire Intangible Assets	$ 400
Acquired technology
Gross carrying amount and accumulated amortization of acquired identifiable intangible assets
Weighted Average Amortization Period	10 years		9 years
Gross Carrying Amount	$ 7,757		$ 5,926
Accumulated Amortization	(2,483)		(2,176)
Total	$ 5,274		$ 3,750
Customer relationships
Gross carrying amount and accumulated amortization of acquired identifiable intangible assets
Weighted Average Amortization Period	10 years		10 years
Gross Carrying Amount	$ 260		$ 260
Accumulated Amortization	(136)		(123)
Total	$ 124		$ 137
Trademark
Gross carrying amount and accumulated amortization of acquired identifiable intangible assets
Weighted Average Amortization Period	5 years		5 years
Gross Carrying Amount	$ 60		$ 60
Accumulated Amortization	$ (60)		(57)
Total			$ 3