Amount of increases (decreases) in cash inflow in financing activities due to impact of financing agreement.

No definition available.

Name:	flnc_IncreaseDecreaseInCashInflowFromFinancingActivitiesDueToImpactOfFinancingAgreement
Namespace Prefix:	flnc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of increases (decreases) in cash used in operating activities due to impact of financing agreement.

+ References

No definition available.

+ Details

Name:	flnc_IncreaseDecreaseInCashUsedInOperatingActivitiesDueToImpactOfFinancingAgreement
Namespace Prefix:	flnc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Number of Suppliers Participating in Program

+ References

No definition available.

+ Details

Name:	flnc_NumberOfSuppliersParticipatingInProgram
Namespace Prefix:	flnc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Supply Chain Financing, Extended Payment Terms Period

+ References

No definition available.

+ Details

Name:	flnc_SupplyChainFinancingExtendedPaymentTermsPeriod
Namespace Prefix:	flnc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Supply Chain Financing Guarantee By Related Party

+ References

No definition available.

+ Details

Name:	flnc_SupplyChainFinancingGuaranteeByRelatedParty
Namespace Prefix:	flnc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Supply Chain Financing, Number Of Parties

+ References

No definition available.

+ Details

Name:	flnc_SupplyChainFinancingNumberOfParties
Namespace Prefix:	flnc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22)(b))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(b))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 405
-SubTopic 50
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479468/405-50-50-3

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 405
-SubTopic 50
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (b)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479468/405-50-50-3

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 405
-SubTopic 50
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479468/405-50-50-3

+ Details

Name:	us-gaap_SupplierFinanceProgramLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of obligation for supplier finance program, classified as current.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 405
-SubTopic 50
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (b)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479468/405-50-50-3

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 405
-SubTopic 50
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479468/405-50-50-3

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 405
-SubTopic 50
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479468/405-50-50-4

+ Details

Name:	us-gaap_SupplierFinanceProgramObligationCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Indicates line item in statement of financial position that includes obligation for supplier finance program, classified as current.

+ References

+ Details

Name:	us-gaap_SupplierFinanceProgramObligationCurrentStatementOfFinancialPositionExtensibleEnumeration
Namespace Prefix:	us-gaap_
Data Type:	enum2:enumerationSetItemType
Balance Type:	na
Period Type:	instant

- Definition

Period when payment is expected to be made to finance provider or intermediary in supplier finance program, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_SupplierFinanceProgramPaymentTimingPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=flnc_NewSCFFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RelatedPartyTransactionsByRelatedPartyAxis=srt_AffiliatedEntityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=flnc_SiemensIndustryIncMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Supply Chain Financing (Details) $ in Millions	3 Months Ended	6 Months Ended
Supply Chain Financing (Details) $ in Millions	Mar. 31, 2026 USD ($) item	Mar. 31, 2026 USD ($) item	Aug. 08, 2025 USD ($)	Aug. 06, 2024 USD ($)
Accruals and Provisions
Extended payment terms period		120 days
Number of parties involved \| item	2	2
Number of suppliers \| item	0
Outstanding obligations	$ 70.8	$ 70.8
Supplier Finance Program, Obligation, Current, Statement of Financial Position [Extensible Enumeration]	Other Liabilities, Current	Other Liabilities, Current
Decrease in cash used in operating activities due to impact of the agreements between the Company's suppliers and the New SCF Bank		$ 70.8
Increase in investing cash inflow due to impact of the agreements between the Company's suppliers and the New SCF Bank		$ 70.8
Line of Credit
Accruals and Provisions
Maximum borrowing capacity				$ 150.0
New SCF Facility
Accruals and Provisions
Number of suppliers \| item		2
Revolving Credit Facility \| Line of Credit
Accruals and Provisions
Maximum borrowing capacity				$ 500.0
Revolving Credit Facility \| New SCF Facility \| Line of Credit
Accruals and Provisions
Maximum borrowing capacity			$ 150.0
Affiliated Entity
Accruals and Provisions
Guarantees issued	$ 100.0	$ 100.0
Minimum
Accruals and Provisions
Term of payment	140 days	140 days
Maximum
Accruals and Provisions
Term of payment	180 days	180 days
Siemens Industry \| Affiliated Entity
Accruals and Provisions
Guarantees issued	$ 50.0	$ 50.0