Revenue - Estimated future revenue (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Revenue
Total estimated future revenue	$ 215,672	$ 227,377
Sales of fuel cell systems
Revenue
Total estimated future revenue	16,467	13,478
Sale of hydrogen installations and other infrastructure
Revenue
Total estimated future revenue	10,072	8,281
Services performed on fuel cell systems and related infrastructure
Revenue
Total estimated future revenue	61,596	77,496
Power Purchase Agreements
Revenue
Total estimated future revenue	121,744	122,659
Other
Revenue
Total estimated future revenue	$ 5,793	$ 5,463
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