Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Disaggregation of revenue
Total gross revenue	$ 39,927		$ 69,007
Sales of fuel cell systems
Disaggregation of revenue
Total gross revenue	14,633		21,207
Sale of hydrogen installations and other infrastructure
Disaggregation of revenue
Total gross revenue	5,989		10,873
Services performed on fuel cell systems and related infrastructure
Disaggregation of revenue
Total gross revenue	7,188		13,786
Power purchase agreements
Disaggregation of revenue
Total gross revenue	5,629		11,118
Fuel delivered to customers
Disaggregation of revenue
Total gross revenue	$ 6,488		$ 12,023
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Disaggregation of revenue
Total gross revenue		$ 22,604		$ 37,839
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Sales of fuel cell systems
Disaggregation of revenue
Total gross revenue		5,582		7,400
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Sale of hydrogen installations and other infrastructure
Disaggregation of revenue
Total gross revenue		2,978		3,357
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Services performed on fuel cell systems and related infrastructure
Disaggregation of revenue
Total gross revenue		5,049		10,198
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Power purchase agreements
Disaggregation of revenue
Total gross revenue		4,945		9,256
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Fuel delivered to customers
Disaggregation of revenue
Total gross revenue		3,986		7,477
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Other
Disaggregation of revenue
Total gross revenue		$ 64		$ 151