Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Summary of Significant Accounting Policies
Upfront revenue recognized		$ 0
Capitalized contract costs		100		$ 0
Net cash flows from financing activities		53,216	$ 65,190
Impairment Loss		0
ASU 2016-18
Summary of Significant Accounting Policies
Net cash flows from financing activities		$ 5,000
Decrease in cash and cash equivalents			6,100
Reclassified From Revenue
Summary of Significant Accounting Policies
Prior period amount reclassed	$ 800		2,300
Reclassified From Cost of Revenue
Summary of Significant Accounting Policies
Prior period amount reclassed	$ 1,200		$ 2,700
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%
Total project costs (as a percent)		30.00%
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%
Total project costs (as a percent)		50.00%
Amortization period of capitalized commission fees		10 years