Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Disaggregation of revenue
Net revenue	$ 53,165	$ 34,593	$ 114,812	$ 69,057
Sales of fuel cell systems and related infrastructure
Disaggregation of revenue
Total gross revenue	21,595		40,948
Sale of hydrogen installations and other infrastructure
Disaggregation of revenue
Total gross revenue	15,073		25,153
Services performed on fuel cell systems and related infrastructure
Disaggregation of revenue
Total gross revenue	5,156		16,330
Power purchase agreements
Disaggregation of revenue
Total gross revenue	5,555		16,365
Fuel delivered to customers
Disaggregation of revenue
Total gross revenue	$ 5,786		$ 16,016
Calculated under Revenue Guidance in Effect before Topic 606
Disaggregation of revenue
Net revenue		34,593		69,057
Calculated under Revenue Guidance in Effect before Topic 606 | Sales of fuel cell systems and related infrastructure
Disaggregation of revenue
Total gross revenue		28,427		35,190
Calculated under Revenue Guidance in Effect before Topic 606 | Sale of hydrogen installations and other infrastructure
Disaggregation of revenue
Total gross revenue		9,633		12,717
Calculated under Revenue Guidance in Effect before Topic 606 | Services performed on fuel cell systems and related infrastructure
Disaggregation of revenue
Total gross revenue		2,217		10,496
Calculated under Revenue Guidance in Effect before Topic 606 | Power purchase agreements
Disaggregation of revenue
Total gross revenue		(1,663)		7,593
Calculated under Revenue Guidance in Effect before Topic 606 | Fuel delivered to customers
Disaggregation of revenue
Total gross revenue		(4,149)		2,782
Calculated under Revenue Guidance in Effect before Topic 606 | Other
Disaggregation of revenue
Total gross revenue		$ 128		$ 279