Revenue - Estimated future revenue (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Disaggregation of revenue
Total estimated future revenue	$ 217,907
Sales of fuel cell systems and related infrastructure
Disaggregation of revenue
Total estimated future revenue	21,029
Sale of hydrogen installations and other infrastructure
Disaggregation of revenue
Total estimated future revenue	6,705
Services performed on fuel cell systems and related infrastructure
Disaggregation of revenue
Total estimated future revenue	75,416
Power purchase agreements
Disaggregation of revenue
Total estimated future revenue	$ 114,757
Maximum | Sales of fuel cell systems and related infrastructure
Disaggregation of revenue
Duration of estimated revenue expexted to be recognized in future (in years)	1 year
Maximum | Services performed on fuel cell systems and related infrastructure
Disaggregation of revenue
Duration of estimated revenue expexted to be recognized in future (in years)	7 years
Maximum | Power purchase agreements
Disaggregation of revenue
Duration of estimated revenue expexted to be recognized in future (in years)	7 years
Minimum | Services performed on fuel cell systems and related infrastructure
Disaggregation of revenue
Duration of estimated revenue expexted to be recognized in future (in years)	5 years
Minimum | Power purchase agreements
Disaggregation of revenue
Duration of estimated revenue expexted to be recognized in future (in years)	5 years
Calculated under Revenue Guidance in Effect before Topic 606
Disaggregation of revenue
Total estimated future revenue		$ 249,305
Calculated under Revenue Guidance in Effect before Topic 606 | Sales of fuel cell systems and related infrastructure
Disaggregation of revenue
Total estimated future revenue		26,298
Calculated under Revenue Guidance in Effect before Topic 606 | Sale of hydrogen installations and other infrastructure
Disaggregation of revenue
Total estimated future revenue		15,512
Calculated under Revenue Guidance in Effect before Topic 606 | Services performed on fuel cell systems and related infrastructure
Disaggregation of revenue
Total estimated future revenue		77,453
Calculated under Revenue Guidance in Effect before Topic 606 | Power purchase agreements
Disaggregation of revenue
Total estimated future revenue		$ 130,042