Consolidated Investment Products - Narrative (Details) $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
Mar. 31, 2021 USD ($) CIPs	Mar. 31, 2021 USD ($) CIPs	Sep. 30, 2020 USD ($) CIPs
Consolidated Investment Products [Abstract]
Number of consolidated investment products \| CIPs	74	74	72
Number of CLOs \| CIPs	5	5	4
Schedule Of Consolidated Investment Products [Line Items]
CIPs' unfunded commitments	$ 10.8	$ 10.8	$ 9.5
Debt, unpaid principal balance	3,486.5	3,486.5	3,017.1
Investments ninety days or more past due [Line Items]	1.8	1.8	$ 3.9
Redeemable Hedge Fund [Member]
Schedule Of Consolidated Investment Products [Line Items]
Alternative Investment	146.5	146.5
Redeemable Private Equity Fund [Member]
Schedule Of Consolidated Investment Products [Line Items]
Alternative Investment	4.0	$ 4.0
Nonredeemable Real Estate and Private Equity Funds [Member]
Schedule Of Consolidated Investment Products [Line Items]
Liquidation weighted-average period		2 years 6 months	1 year 10 months 24 days
Alternative Investment	58.7	$ 58.7	$ 51.2
CIPs [Member]
Schedule Of Consolidated Investment Products [Line Items]
Liquidation weighted-average period		3 years 3 months 18 days	4 years 2 months 12 days
CIPs' unfunded commitments	44.9	$ 44.9	$ 94.0
Unfunded commitments Company contractually obligated to fund	5.6	5.6	11.4
Debt, unpaid principal balance	1,759.5	1,759.5	1,333.4
CIPs [Member] \| Nonredeemable Real Estate and Private Equity Funds [Member]
Schedule Of Consolidated Investment Products [Line Items]
Alternative Investment	$ 375.6	$ 375.6	$ 261.1
CIPs [Member] \| Minimum [Member]
Schedule Of Consolidated Investment Products [Line Items]
Stated interest rate	1.00%	1.00%	1.00%
CIPs [Member] \| Maximum [Member]
Schedule Of Consolidated Investment Products [Line Items]
Stated interest rate	4.45%	4.45%	5.81%
Collateralized loan obligations [Member]
Schedule Of Consolidated Investment Products [Line Items]
Net gains related to its own economic interests	$ 6.5	$ 7.4
Debt face amount	1,649.0	1,649.0	$ 1,235.8
Collateralized loan obligations [Member] \| CIPs [Member]
Schedule Of Consolidated Investment Products [Line Items]
Debt, unpaid principal balance	$ 1,649.6	$ 1,649.6	$ 1,179.7
Collateralized loan obligations [Member] \| CIPs [Member] \| Minimum [Member]
Schedule Of Consolidated Investment Products [Line Items]
Stated interest rate	1.00%	1.00%	1.48%
Collateralized loan obligations [Member] \| CIPs [Member] \| Maximum [Member]
Schedule Of Consolidated Investment Products [Line Items]
Stated interest rate	8.15%	8.15%	8.19%

No definition available.

Name:	ben_ConsolidatedInvestmentProductsAbstract
Namespace Prefix:	ben_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Gains Losses On Assets And Liabilities Of Consolidated Vies Net

+ References

No definition available.

+ Details

Name:	ben_GainsLossesOnAssetsAndLiabilitiesOfConsolidatedViesNet
Namespace Prefix:	ben_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Investments, Ninety Or More Days Past Due.

+ References

No definition available.

+ Details

Name:	ben_InvestmentsNinetyOrMoreDaysPastDue
Namespace Prefix:	ben_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Number of consolidated (deconsolidated) collateralized loan obligations.

+ References

No definition available.

+ Details

Name:	ben_NumberOfCollateralizedObligationConsolidatedDeconsolidated
Namespace Prefix:	ben_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Refers to number of consolidated investment products at period end.

+ References

No definition available.

+ Details

Name:	ben_NumberOfConsolidatedInvestmentproducts
Namespace Prefix:	ben_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

[Line Items] for Tabular disclosure of qualitative and quantitative information related to investment products holds, whether or not such investment products (CIP) is included in the reporting entity's consolidated financial statements. Includes, but is not limited to, description of the significant judgments and assumptions made in determining whether a CIP (as defined) held by the entity requires the CIP (as defined) to be consolidated and (or) disclose information about its involvement with the CIP, individually or in aggregate (as applicable); the nature of restrictions, if any, on the CIP's assets and on the settlement of its liabilities reported by an entity in its statement of financial position, including the carrying amounts of such assets and liabilities; the nature of, and changes in, the risks associated with involvement in the CIP; how involvement with the CIP affects the entity's financial position, financial performance, and cash flows; the lack of recourse if creditors (or beneficial interest holders) of the CIP have no recourse to the general credit of the primary beneficiary (if applicable); the terms of arrangements, giving consideration to both explicit arrangements and implicit variable interests, if any, that could require the entity to provide financial support to the SIP, including events or circumstances that could expose the entity to a loss; the methodology used by the entity for determining whether or not it is the primary beneficiary of the consolidated investment products; the significant factors considered and judgments made in determining that the power to direct the activities of a CIP that most significantly impact the CIP's economic performance are shared (as defined); the carrying amounts and classification of assets and liabilities of the CIP included in the statement of financial position.

+ References

No definition available.

+ Details

Name:	ben_ScheduleOfConsolidatedInvestmentProductsLineItems
Namespace Prefix:	ben_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Portion of unfunded commitments that the company was contractually obligated to fund.

+ References

No definition available.

+ Details

Name:	ben_UnfundedCommitmentsCompanyContractuallyObligatedToFundConsolidatedInvestmentProducts
Namespace Prefix:	ben_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Fair value of investment other than investment in equity security, investment in debt security and equity method investment. Includes, but is not limited to, investment in certain entities that calculate net asset value per share. Example includes, but is not limited to, investment in hedge fund, venture capital fund, private equity fund, and real estate partnership or fund.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=SL6742756-110258

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 35
-Paragraph 54B
-URI http://asc.fasb.org/extlink&oid=117332851&loc=SL7495116-110257

+ Details

Name:	us-gaap_AlternativeInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=119993939&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Remaining period of expected liquidation by investee of underlying asset of investment measured at net asset value per share or unit. In 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareLiquidatingInvestmentRemainingPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The amount of unfunded commitments for which the entity is obligated, to those certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 6A
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=SL6742756-110258

+ Details

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

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=120520924&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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

- Details

Name:	us-gaap_InvestmentTypeAxis=ben_RedeemableHedgeFundMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=ben_RedeemablePrivateEquityFundMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=ben_NonredeemableFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ConsolidatedEntitiesAxis=ben_ConsolidatedInvestmentProductsMember
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:	us-gaap_UnderlyingAssetClassAxis=us-gaap_CollateralizedLoanObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: