Revenue - Estimated future revenue (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Revenue
Total estimated future revenue	$ 237,842	$ 223,031
Sales of fuel cell systems
Revenue
Total estimated future revenue	25,954	21,029
Sale of hydrogen installations and other infrastructure
Revenue
Total estimated future revenue	11,448	6,705
Services performed on fuel cell systems and related infrastructure
Revenue
Total estimated future revenue	60,196	73,484
Power Purchase Agreements
Revenue
Total estimated future revenue	134,871	114,757
Other
Revenue
Total estimated future revenue	$ 5,373	$ 7,056
Maximum | Sales of fuel cell systems
Revenue
Duration of estimated revenue expected to be recognized in future (in years)	1 year
Maximum | Services performed on fuel cell systems and related infrastructure
Revenue
Duration of estimated revenue expected to be recognized in future (in years)	7 years
Maximum | Power Purchase Agreements
Revenue
Duration of estimated revenue expected to be recognized in future (in years)	7 years
Minimum | Services performed on fuel cell systems and related infrastructure
Revenue
Duration of estimated revenue expected to be recognized in future (in years)	5 years
Minimum | Power Purchase Agreements
Revenue
Duration of estimated revenue expected to be recognized in future (in years)	5 years