Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	1 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018	Mar. 31, 2018
Summary of Significant Accounting Policies
Impairment Loss		$ 0
Upfront revenue recognized		0
Principal amount	$ 100,000	100,000
Proceeds from issuance of debt		$ 39,052	$ 95,856
Minimum
Summary of Significant Accounting Policies
Payment terms for fuel cells and its services		30 days
Extension period		5 years
Uptime of the fleet (as a percent)		97.00%
Amortization period of capitalized commission fees		5 years
Maximum
Summary of Significant Accounting Policies
Payment terms for fuel cells and its services		90 days
Extension period		10 years
Uptime of the fleet (as a percent)		98.00%
Amortization period of capitalized commission fees		10 years
Convertible Senior Notes
Summary of Significant Accounting Policies
Convertible Notes Payable	48,000	$ 48,000
Principal amount	$ 40,000	$ 40,000
Interest rate (as a percent)	7.50%	7.50%
Proceeds from issuance of debt		$ 39,100
$40M Convertible Senior Note
Summary of Significant Accounting Policies
Principal amount	$ 40,000	$ 40,000
Interest rate (as a percent)	7.50%	7.50%
Proceeds from issuance of debt	$ 39,100
Percentage of principal amount to be repaid at maturity	120.00%
Maturity principal amount	$ 48,000	$ 48,000
Unamortized debt discount	8,000	8,000
$100M Convertible Senior Notes
Summary of Significant Accounting Policies
Principal amount	$ 100,000	$ 100,000		$ 100,000
Interest rate (as a percent)	5.50%	5.50%		5.50%
Proceeds from issuance of debt		$ 52,356
Percentage of principal amount to be repaid at maturity	100.00%
Unamortized debt discount	$ 29,605	$ 29,605