Revenue - Estimated future revenue (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Revenue
Total estimated future revenue	$ 216,171	$ 218,005
Sales of fuel cell systems
Revenue
Total estimated future revenue	26,354	17,318
Sale of hydrogen installations and other infrastructure
Revenue
Total estimated future revenue	9,072	9,141
Services performed on fuel cell systems and related infrastructure
Revenue
Total estimated future revenue	68,878	73,381
Power Purchase Agreements
Revenue
Total estimated future revenue	105,441	111,533
Other
Revenue
Total estimated future revenue	$ 6,427	$ 6,633
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