Debt and Financing Obligation - Additional Information (Details)													1 Months Ended			3 Months Ended	6 Months Ended	12 Months Ended
	Jul. 12, 2019 USD ($)	Apr. 30, 2019 USD ($)	Apr. 01, 2019 USD ($)	Mar. 29, 2019 USD ($)	Feb. 28, 2019 USD ($) MW	Jan. 09, 2019 USD ($)	Dec. 31, 2018	Dec. 21, 2018 USD ($) MW	Dec. 13, 2018 USD ($) MW	Sep. 30, 2018	Jun. 30, 2018	Mar. 28, 2018 USD ($)	Apr. 30, 2019 USD ($)	Jan. 31, 2019 USD ($) Employee Position	Nov. 30, 2015 USD ($) Position	Jan. 31, 2018	Apr. 30, 2019 USD ($)	Oct. 31, 2017	Apr. 01, 2020 USD ($)	May 01, 2019	Oct. 31, 2018 USD ($)	Oct. 01, 2018 USD ($)	Nov. 30, 2016 USD ($)	Apr. 30, 2016 USD ($)
Debt Instrument [Line Items]
Lease payment term		36 months											36 months				36 months
Minimum [Member]
Debt Instrument [Line Items]
Optional project financing term																	5 years
Connecticut Green Bank Note [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 5,900,000											$ 5,900,000				$ 5,900,000
Debt Instrument, Interest Rate, Stated Percentage		5.00%											5.00%				5.00%
Long-term Line of Credit		$ 6,051,000											$ 6,051,000				$ 6,051,000				$ 6,052,000
PNC Energy Capital, LLC [Member]
Debt Instrument [Line Items]
Long-term Line of Credit		45,300,000											45,300,000				$ 45,300,000				46,100,000
Term of loan																	10 years
State of Connecticut [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage															2.00%
Debt Instrument, Face Amount		$ 10,000,000											10,000,000		$ 10,000,000		$ 10,000,000				10,000,000
DebtInstrument term															15 years
Debt instrument, payment terms, period principal payments are deferred															4 years
Debt instrument, date of first required payment															Dec. 01, 2019
Loan forgiveness terms																	if the Company created 165 full-time positions and retained 538 full-time positions for two consecutive years (the “Employment Obligation”) as measured on October 28, 2017 (the “Target Date”). The Assistance Agreement was subsequently amended in April 2017 to extend the Target Date by two years to October 28, 2019
State of Connecticut [Member] | Employment Obligation [Member]
Debt Instrument [Line Items]
Number of full time positions required to be created | Position															165
Number of full time positions required to be retained | Position															538
State of Connecticut [Member] | Employment Obligation [Member] | Second Amendment [Member]
Debt Instrument [Line Items]
Number of full time positions required to be maintained | Employee														538
Number of consecutive months maintain the full time positions														24 months
Additional number of full time positions required to be create | Position														91
Additional credits to be earned														$ 2,000,000
Principal payable number of employee under employee obligation target														$ 18,587.36
State of Connecticut [Member] | Maximum [Member]
Debt Instrument [Line Items]
Percentage of loan forgiveness		50.00%
Hercules Capital, Inc. [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity																								$ 25,000,000
Long-term Line of Credit		$ 23,812,000											23,812,000				$ 23,812,000				25,343,000
Term of loan																	30 months
Debt Instrument, Interest Rate, Stated Percentage Rate Range							10.90%			10.65%	10.40%	10.15%					9.90%
Other Deductions and Charges																					1,700,000	$ 1,700,000
Loan Advance												$ 13,100,000
Maturity date																	Apr. 01, 2020
Debt instrument, basis spread on variable rate																	4.50%
Principal payments amortizing per month			$ 1,800,000
Cash covenant minimum required percentage of unrestricted cash balance on outstanding loan balance and accounts payable			75.00%	50.00%	50.00%															75.00%
Hercules Capital, Inc. [Member] | Series C Preferred Stock [Member]
Debt Instrument [Line Items]
Debt instrument, amendment description		The Hercules Agreement, as amended, contains certain representations and warranties, affirmative and negative covenants, and events of default, including the occurrence of a circumstance that would reasonably be expected to have a material adverse effect, that entitle Hercules to cause the indebtedness under the agreement to become immediately due and payable. The occurrence of an event of default under the Hercules Agreement also constitute or may result in an event of default under, and cause or may cause the acceleration of a number of material financial obligations including the State of Connecticut loan, the Connecticut Green Bank Loan and the Generate Lending, PNC and Fifth Third project finance facilities. The occurrence of an event of default under the Hercules Agreement also constitute a triggering event under the Series D Certificate of Designations. As collateral for obligations under the Hercules Agreement, the Company granted Hercules a security interest in FuelCell Energy, Inc.’s existing and thereafter-acquired assets except for intellectual property and certain other excluded assets. The collateral does not include assets held by FuelCell Energy Finance, LLC (“FuelCell Finance”) or any project subsidiary thereof. The Company may continue to collateralize and finance its project subsidiaries through other lenders and partners.
Hercules Capital, Inc. [Member] | Scenario Forecast [Member]
Debt Instrument [Line Items]
Other Deductions and Charges																			$ 900,000
Hercules Capital, Inc. [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage Rate Range																10.00%
Hercules Capital, Inc. [Member] | Minimum [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage Rate Range																		9.75%
Webster Bank Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage Rate Range																	5.00%
Notes Payable to Bank, Current		$ 800,000											800,000				$ 800,000				$ 1,100,000		$ 2,300,000
NRG Energy [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		40,000,000											40,000,000				40,000,000
Long-term Line of Credit		5,750,000							$ 0				5,750,000				$ 5,750,000
Non-refundable fee paid				$ 250,000
NRG Energy [Member] | Tulare BioMAT [Member] | California [Member]
Debt Instrument [Line Items]
Capacity of Plant | MW									2.8
NRG Energy [Member] | Central CA Fuel Cell 2, LLC [Member]
Debt Instrument [Line Items]
Loan Advance									$ 5,800,000
NRG Energy [Member] | Scenario Forecast [Member]
Debt Instrument [Line Items]
Non-refundable fee paid	$ 750,000
NRG Energy [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage Rate Range																	8.50%
NRG Energy [Member] | Minimum [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage Rate Range																	8.00%
Generate Lending Construction Loan Agreement [Member]
Debt Instrument [Line Items]
Long-term Line of Credit		10,000,000											10,000,000				$ 10,000,000
Generate Lending Construction Loan Agreement [Member] | FuelCell Energy Finance II, LLC [Member] | Generate Lending, LLC [Member]
Debt Instrument [Line Items]
Loan Advance								$ 10,000,000
Debt instrument, basis spread on variable rate								9.50%
Percentage of construction budget								100.00%
Unlevered internal rate of return percentage to lender								10.00%
Availability period for working capital loans								36 months
Equity ownership								100.00%
Purchase price adjustment								50.00%
Draw down fee percentage								3.00%
Disposition fee percentage of total sale price								3.00%
Generate Lending Construction Loan Agreement [Member] | California [Member] | FuelCell Energy Finance II, LLC [Member] | Generate Lending, LLC [Member] | Bolthouse Farms [Member]
Debt Instrument [Line Items]
Capacity of Plant | MW								5
Generate Lending Construction Loan Agreement [Member] | Maximum [Member] | FuelCell Energy Finance II, LLC [Member] | Generate Lending, LLC [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity								$ 300,000,000
Line of credit facility, current borrowing capacity								$ 100,000,000
Generate Lending Construction Loan Agreement [Member] | Minimum [Member] | FuelCell Energy Finance II, LLC [Member] | Generate Lending, LLC [Member]
Debt Instrument [Line Items]
Unlevered internal rate of return percentage to lender								10.00%
After-tax yield percentage								12.00%
Tranche One [Member]
Debt Instrument [Line Items]
Aggregate working capital loan amount minimum limit								$ 100,000,000
Aggregate working capital loan amount maximum limit								$ 200,000,000
Disposition fee percentage								2.00%
Tranche Two [Member]
Debt Instrument [Line Items]
Aggregate working capital loan amount minimum limit								$ 200,000,000
Aggregate working capital loan amount maximum limit								$ 300,000,000
Disposition fee percentage								1.00%
Enhanced Capital Loan and Security Agreement [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity						$ 1,500,000
Long-term Line of Credit		1,500,000											1,500,000				1,500,000
Debt instrument, basis spread on variable rate						6.00%
Debt instrument, maturity period description						The loan maturity date is three years from the date of the Enhanced Capital Loan Agreement upon which the outstanding principal and accrued interest will be payable.
Debt instrument, maturity period						3 years
New Construction Loan Facility [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity					$ 23,000,000
Loan Advance					$ 9,700,000								1,400,000
Maturity date					Oct. 31, 2019
Total outstanding balance		$ 11,100,000											$ 11,100,000				$ 11,100,000
New Construction Loan Facility [Member] | one-month LIBOR [Member]
Debt Instrument [Line Items]
Debt instrument, basis spread on variable rate					2.25%
New Construction Loan Facility [Member] | Groton CT [Member]
Debt Instrument [Line Items]
Capacity of Plant | MW					7.4