Revenue - Estimated future revenue (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Disaggregation of revenue
Total estimated future revenue	$ 221,914
Sales of fuel cell systems
Disaggregation of revenue
Total estimated future revenue	13,478
Sale of hydrogen installations and other infrastructure
Disaggregation of revenue
Total estimated future revenue	8,281
Services performed on fuel cell systems and related infrastructure
Disaggregation of revenue
Total estimated future revenue	77,496
Power purchase agreements
Disaggregation of revenue
Total estimated future revenue	$ 122,659
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Disaggregation of revenue
Total estimated future revenue		$ 260,931
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Sales of fuel cell systems
Disaggregation of revenue
Total estimated future revenue		26,298
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | 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 [Member] | Services performed on fuel cell systems and related infrastructure
Disaggregation of revenue
Total estimated future revenue		89,079
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Power purchase agreements
Disaggregation of revenue
Total estimated future revenue		$ 130,042