Consolidated Investment Products - Schedule of Changes in Level 3 Assets of CIPs (Details) - CIPs [Member] - Level 3 [Member] - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2021	Sep. 30, 2020
Schedule Of Consolidated Investment Products [Line Items]
Balance at beginning of year - assets	$ 686.4	$ 503.1
Acquisition		37.9
Realized and unrealized gains (losses) included in investment and other income of consolidated investment products, net	131.4	(54.2)
Purchases	267.4	238.9
Sales and settlements	(67.5)	(12.5)
Deconsolidations	(485.0)	(47.8)
Transfers into Level 3 - assets	21.2	2.2
Transfers out of level 3 - assets	(1.9)	(1.1)
Foreign exchange revaluation	11.2	19.9
Balance at End of Year - assets	563.2	686.4
Change in unrealized losses included in net income relating to assets held at end of year	129.8	(54.2)
Equity and debt securities [Member]
Schedule Of Consolidated Investment Products [Line Items]
Balance at beginning of year - assets	322.3	333.8
Acquisition		0.0
Realized and unrealized gains (losses) included in investment and other income of consolidated investment products, net	122.8	(47.2)
Purchases	105.7	84.0
Sales and settlements	(62.8)	(5.0)
Deconsolidations	(36.2)	(47.8)
Transfers into Level 3 - assets	2.7	2.2
Transfers out of level 3 - assets	(1.9)	(1.1)
Foreign exchange revaluation	0.7	3.4
Balance at End of Year - assets	453.3	322.3
Change in unrealized losses included in net income relating to assets held at end of year	123.9	(47.2)
Real estate [Member]
Schedule Of Consolidated Investment Products [Line Items]
Balance at beginning of year - assets	339.2	152.7
Acquisition		20.3
Realized and unrealized gains (losses) included in investment and other income of consolidated investment products, net	8.3	(5.2)
Purchases	161.7	154.9
Sales and settlements	0.0	0.0
Deconsolidations	(448.8)	0.0
Transfers into Level 3 - assets	18.5	0.0
Transfers out of level 3 - assets	0.0	0.0
Foreign exchange revaluation	10.5	16.5
Balance at End of Year - assets	89.4	339.2
Change in unrealized losses included in net income relating to assets held at end of year	5.4	(5.2)
Loans [Member]
Schedule Of Consolidated Investment Products [Line Items]
Balance at beginning of year - assets	24.9	16.6
Acquisition		17.6
Realized and unrealized gains (losses) included in investment and other income of consolidated investment products, net	0.3	(1.8)
Purchases	0.0	0.0
Sales and settlements	(4.7)	(7.5)
Deconsolidations	0.0	0.0
Transfers into Level 3 - assets	0.0	0.0
Transfers out of level 3 - assets	0.0	0.0
Foreign exchange revaluation	0.0	0.0
Balance at End of Year - assets	20.5	24.9
Change in unrealized losses included in net income relating to assets held at end of year	$ 0.5	$ (1.8)

This element represents the effect of exchange rate changes on the fair value measurement.

No definition available.

Name:	ben_FairValueMeasurementEffectOfExchangeRateChanges
Namespace Prefix:	ben_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of increase (decrease) of assets measured at fair value on a recurring basis using unobservable inputs acquired during the period.

+ References

No definition available.

+ Details

Name:	ben_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetAcquisition
Namespace Prefix:	ben_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of sale and settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

No definition available.

+ Details

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

- Definition

Fair Value Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Consolidation (Deconsolidation)

+ References

No definition available.

+ Details

Name:	ben_FairValueMeasurementwithUnobservableInputsReconciliationRecurringBasisAssetsConsolidationDeconsolidation
Namespace Prefix:	ben_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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

Amount of unrealized gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3) and still held.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of gain (loss) recognized in income from asset measured at fair value on recurring basis using unobservable input (level 3).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

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

- Details

Name:	srt_ConsolidatedEntitiesAxis=ben_ConsolidatedInvestmentProductsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=ben_DebtAndEquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_RealEstateInvestmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=us-gaap_LoansReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: