Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Summary of Significant Accounting Policies
Impairment Loss	$ 0
Upfront revenue recognized	$ 0
Reclassified From Revenue
Summary of Significant Accounting Policies
Prior period amount reclassed		$ 800
Reclassified From Cost of Revenue
Summary of Significant Accounting Policies
Prior period amount reclassed		$ 1,100
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