Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue:
Sales of fuel cell systems and related infrastructure	$ 15,352	$ 45,179	$ 8,560	$ 2,197	$ 19,993	$ 5,653	$ 9,121	$ 5,218	$ 71,288	$ 39,985	$ 78,002
Services performed on fuel cell systems and related infrastructure	6,734	5,842	5,049	5,149	5,060	4,763	5,360	5,273	22,774	20,456	14,012
Power Purchase Agreements	5,597	5,428	4,945	4,311	4,061	3,858	3,062	2,706	30,641	16,132	5,253
Fuel delivered to customers	5,975	4,850	3,986	3,491	3,359	2,909	2,638	2,010	18,302	10,916	5,075
Other	5	128	64	87	105	376	278	125	284	884	481
Gross revenue	33,663	61,427	22,604	15,235	32,578	17,559	20,459	15,332	132,929	85,928	103,288
Provision for common stock warrants	(1,790)	(26,057)	(1,820)						(29,667)
Net revenue	31,873	35,370	20,784	15,235					103,262	85,928	103,288
Gross (loss) profit	(657)	(19,410)	(3,543)	(4,479)	3,011	381	384	170	(28,089)	3,946	(9,890)
Operating expenses	17,060	16,971	24,529	15,143	14,948	13,637	13,760	13,120	73,703	55,465	49,112
Operating loss	(17,717)	(36,381)	(28,072)	(19,622)	(11,937)	(13,256)	(13,376)	(12,950)	(101,792)	(51,519)	(59,002)
Net loss attributable to common shareholders	$ (19,451)	$ (41,008)	$ (42,645)	$ (27,074)	$ (19,237)	$ (13,420)	$ (13,154)	$ (11,780)	$ (130,178)	$ (57,591)	$ (55,795)
Loss per share:
Basic and diluted (in dollars per share)	$ (0.09)	$ (0.18)	$ (0.19)	$ (0.14)	$ (0.11)	$ (0.07)	$ (0.07)	$ (0.07)	$ (0.60)	$ (0.32)	$ (0.32)
Loss on Contracts							$ 1,100
Tranche one of warrants issued with the Amazon.com, Inc transaction agreement
Revenue:
Operating expenses			$ 7,100