Debt and Financing Obligation - Additional Information (Details)															1 Months Ended			3 Months Ended	9 Months Ended
	Sep. 30, 2019 USD ($)	Jul. 25, 2019 USD ($)	Jul. 19, 2019 MW	Jun. 26, 2019 USD ($)	Jun. 11, 2019 USD ($)	Jun. 03, 2019	May 09, 2019 USD ($)	Apr. 01, 2019 USD ($)	Mar. 29, 2019 USD ($)	Feb. 28, 2019 USD ($) MW	Jan. 09, 2019 USD ($)	Dec. 21, 2018 USD ($) MW	Dec. 13, 2018 USD ($) MW	Jun. 13, 2018	Apr. 30, 2019 USD ($)	Jan. 31, 2019 USD ($) Employee Position	Nov. 30, 2015 USD ($) Position	Jul. 31, 2019 USD ($)	Jul. 31, 2019 USD ($)	Apr. 01, 2020 USD ($)	Oct. 31, 2018 USD ($)	Oct. 01, 2018 USD ($)	Apr. 30, 2016 USD ($)
Debt Instrument [Line Items]
Long-term Line of Credit																		$ 1,800,000	$ 1,800,000
Interest rate, basis points							275.00%
Amendment period description																			“Amendment Period” is defined as the period from and after July 24, 2019 through the earlier of (i) September 30, 2019 and (ii) the occurrence of any event of default under the Tenth Hercules Amendment; provided, however, that in the event that the outstanding balance of the secured obligations (including accrued interest, fees, costs, and charges) has been paid down to an amount that is less than or equal to $5.0 million on or before September 30, 2019, the Amendment Period shall be extended automatically through the earlier of (i) October 22, 2019 and (ii) the occurrence of any event of default under the Tenth Hercules Amendment.
EMRE [Member]
Debt Instrument [Line Items]
License agreement payment					$ 10,000,000
B. Riley FBR, Inc and Oppenheimer & Co. Inc. [Member] | At Market Issuance Sales Agreement [Member]
Debt Instrument [Line Items]
Percentage on net proceeds after deducting commission and offering related expense														30.00%
Maximum [Member]
Debt Instrument [Line Items]
Repayments of secured obligations for automatic extension of amendment period	$ 5,000,000
Connecticut Green Bank Note [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity																		5,900,000	$ 5,900,000
Subordinated Credit Agreement [Member] | Bridgeport Fuel Cell, LLC [Member]
Debt Instrument [Line Items]
Long-term Line of Credit																		$ 5,900,000	$ 5,900,000
Proceeds from long-term lines of credit							$ 6,000,000
Debt instrument, Interest rate							8.00%
Debt instrument, covenant terms																			On May 9, 2019, in connection with the closing of the purchase of BFC, BFC entered into a subordinated credit agreement with the Connecticut Green Bank whereby Connecticut Green Bank provided financing in the amount of $6.0 million (the “Subordinated Credit Agreement”). As security for the Subordinated Credit Agreement, Connecticut Green Bank received a perfected lien, subordinated and second in priority to the liens securing the $25.0 million loaned under the BFC Credit Agreement (as defined below), in all of the same collateral securing the BFC Credit Agreement. The interest rate under the Subordinated Credit Agreement is 8% per annum. Principal and interest are due monthly in amounts sufficient to fully amortize the loan over an 84 month period ending in May 2026. The Subordinated Credit Agreement contains customary representations, warranties and covenants. The balance under the Subordinated Credit Agreement as of July 31, 2019 was $5.9 million.
Subordinated Credit Agreement [Member] | Bridgeport Fuel Cell, LLC [Member] | Senior Term Loan [Member]
Debt Instrument [Line Items]
Liens securing senior term loan amount							$ 25,000,000
Credit Agreement [Member]
Debt Instrument [Line Items]
Maturity date							May 09, 2025
Interest rate description																		The interest rate under the BFC Credit Agreement fluctuates monthly at the 30-day LIBOR rate plus 275 basis points
Debt instrument, interest rate basis term							30 days
Credit Agreement [Member] | Interest Rate Swap [Member]
Debt Instrument [Line Items]
Net interest rate							5.09%
Credit Agreement [Member] | Interest Rate Swap [Member] | Level Adjustments [Member]
Debt Instrument [Line Items]
Mark-to-market adjustment																		$ 400,000	$ 400,000
Credit Agreement [Member] | Bridgeport Fuel Cell, LLC [Member]
Debt Instrument [Line Items]
Debt instrument, covenant terms							The BFC Credit Agreement also requires BFC to maintain a debt service reserve at each of Liberty Bank and Fifth Third of $1.25 million, which debt service reserves were funded on May 10, 2019, to be held in deposit accounts at each respective bank, with funds to be disbursed with the consent of or at the request of the required lenders in their sole discretion. Each of Liberty Bank and Fifth Third also has an operation and module replacement reserve (“O&M Reserve”) of $250.0 thousand, both of which were funded at closing, to be held in deposit accounts at each respective bank, and thereafter BFC is required to deposit $100.0 thousand per month into each O&M Reserve for the first five years of the BFC Credit Agreement, with such funds to be released at the sole discretion of Liberty Bank and Fifth Third, as applicable. BFC is also required to maintain excess cash flow reserve accounts at each of Liberty Bank and Fifth Third and to deposit 50% of the excess cash flows from the Bridgeport Fuel Cell Project into these accounts. Excess cash flow consists of cash generated by BFC from the Bridgeport Fuel Cell Project after payment of all expenses (including after payment of service fees to the Company), debt service to Liberty Bank and Fifth Third, the funding of all required reserves, and payments to Connecticut Green Bank for the subordinated facility. BFC is also required to maintain a debt service coverage ratio of not less than 1.20, measured annually based on fiscal quarters beginning with the quarter ended July 31, 2020. The BFC Credit Agreement contains other representations, warranties and covenants and includes a material adverse effect clause related to the operations, business, properties, liabilities or prospects of Bridgeport Fuel Cell, LLC.
Principal and interest frequency of periodic payment							monthly
Loans amortization period							84 months
Credit Agreement [Member] | Bridgeport Fuel Cell, LLC [Member] | Minimum [Member]
Debt Instrument [Line Items]
Debt service coverage ratio							120.00%
Credit Agreement [Member] | Bridgeport Fuel Cell, LLC [Member] | Fifth Third Bank [Member]
Debt Instrument [Line Items]
Proceeds from long-term lines of credit							$ 12,500,000
Debt service reserves required to be maintained and held in deposit accounts							1,250,000
Operation and module replacement reserves required to be maintained and held in deposit accounts							250,000
Operation and module replacement reserve to be deposited per month							$ 100,000
Percentage of excess cash flows to be maintained in accounts							50.00%
Credit Agreement [Member] | Bridgeport Fuel Cell, LLC [Member] | Liberty Bank [Member]
Debt Instrument [Line Items]
Proceeds from long-term lines of credit							$ 12,500,000
Debt service reserves required to be maintained and held in deposit accounts							1,250,000
Operation and module replacement reserves required to be maintained and held in deposit accounts							250,000
Operation and module replacement reserve to be deposited per month							$ 100,000
Percentage of excess cash flows to be maintained in accounts							50.00%
PNC Energy Capital, LLC [Member]
Debt Instrument [Line Items]
Long-term Line of Credit																		45,200,000	$ 45,200,000		$ 46,100,000
Term of loan																			10 years
State of Connecticut [Member]
Debt Instrument [Line Items]
Debt instrument, Interest rate																	2.00%
Debt Instrument, Face Amount																	$ 10,000,000	10,000,000	$ 10,000,000		10,000,000
Debt Instrument 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] | Maximum [Member]
Debt Instrument [Line Items]
Percentage of loan forgiveness																			50.00%
State of Connecticut [Member] | Second Amendment [Member] | Employment Obligation [Member]
Debt Instrument [Line Items]
Number of minimum 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
Hercules Capital, Inc. [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity																							$ 25,000,000
Long-term Line of Credit																		$ 7,426,000	$ 7,426,000		$ 25,343,000
Debt instrument, Interest rate																		10.65%	10.65%
Maturity date																			Oct. 22, 2019
Interest rate, basis points																			4.50%
Term of loan																			30 months
Debt Instrument, Interest Rate, Stated Percentage Rate Range																			9.90%
Other Deductions and Charges																						$ 1,700,000
Principal payments expected amortizing per month								$ 1,800,000
Repayment of loan		$ 4,000,000			$ 1,400,000
Additional default interest rate per annum						5.00%
Accrued and unpaid default rate interest																		$ 100,000	$ 100,000
Hercules Capital, Inc. [Member] | Series C Preferred Stock [Member]
Debt Instrument [Line Items]
Debt instrument, amendment description																			The Hercules Agreement 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 constitutes or may result in an event of default under, and causes or may cause the acceleration of, a number of material financial obligations of the Company, including the loan from the State of Connecticut, the loan from Connecticut Green Bank, and the project finance facilities with Generate Lending, PNC and Fifth Third. The occurrence of an event of default under the Hercules Agreement also constitutes 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 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] | EMRE [Member]
Debt Instrument [Line Items]
Repayment of loan				$ 6,000,000
Hercules Capital, Inc. [Member] | Scenario Forecast [Member]
Debt Instrument [Line Items]
Other Deductions and Charges																				$ 900,000
Hercules Capital, Inc. [Member] | Minimum [Member]
Debt Instrument [Line Items]
Maturity date																			Oct. 21, 2019
NRG Energy [Member]
Debt Instrument [Line Items]
Long-term Line of Credit																		5,750,000	$ 5,750,000
Non-refundable fee paid									$ 750,000
Debt instrument, maturity period description																			The Maturity date of each note payable to NRG is now the date that is the earliest of (a) September 30, 2019, (b) the commercial operation date or substantial completion date, as applicable, with respect to the fuel cell project owned by the co-borrower under such note, and (c) the repayment in full or the closing of a refinancing of the Company’s indebtedness with Hercules. NRG may also accelerate the maturity dates if it determines, in its sole discretion, that the deems that the Company is not making sufficient progress towards the completion of the construction of the 2.8 MW Tulare BioMAT projet in California.
NRG Energy [Member] | Central CA Fuel Cell 2, LLC [Member]
Debt Instrument [Line Items]
Loan advance													$ 5,800,000
NRG Energy [Member] | Tulare BioMAT [Member] | California [Member]
Debt Instrument [Line Items]
Capacity of plant | MW			2.8										2.8
Generate Lending Construction Loan Agreement [Member]
Debt Instrument [Line Items]
Long-term Line of Credit																		10,000,000	$ 10,000,000
Generate Lending Construction Loan Agreement [Member] | Generate Lending, LLC [Member] | FuelCell Energy Finance II, LLC [Member]
Debt Instrument [Line Items]
Interest rate, basis points												9.50%
Loan advance												$ 10,000,000
Percentage of construction budget												100.00%
Unlevered internal rate of return percentage to lender												10.00%
Availability period for working capital loans												36 months
Generate Lending Construction Loan Agreement [Member] | Generate Lending, LLC [Member] | California [Member] | FuelCell Energy Finance II, LLC [Member] | Bolthouse Farms [Member]
Debt Instrument [Line Items]
Capacity of plant | MW												5
Generate Lending Construction Loan Agreement [Member] | Generate Lending, LLC [Member] | Maximum [Member] | FuelCell Energy Finance II, LLC [Member]
Debt Instrument [Line Items]
Line of credit facility, current borrowing capacity												$ 100,000,000
Enhanced Capital Term 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
Interest rate, basis points											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
Maturity date										Oct. 31, 2019
Loan advance										$ 9,700,000					$ 1,400,000
Total outstanding balance																		$ 11,100,000	$ 11,100,000
New Construction Loan Facility [Member] | one-month LIBOR [Member]
Debt Instrument [Line Items]
Interest rate, basis points										2.25%
New Construction Loan Facility [Member] | Groton CT [Member]
Debt Instrument [Line Items]
Capacity of plant | MW										7.4