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Debt - Outstanding Debt (Details) - USD ($) $ in Millions	Sep. 30, 2025	Mar. 31, 2025	Sep. 30, 2024
Debt Instrument [Line Items]
Total	$ 2,362.0		$ 2,780.3
Debt issuance costs	(7.5)		(8.6)
Franklin Resources, Inc. [Member]
Debt Instrument [Line Items]
Senior notes	1,196.0		1,595.5
Legg Mason [Member]
Debt Instrument [Line Items]
Total	1,173.5		1,193.4
Notes Due March 2025 [Member] \| Franklin Resources, Inc. [Member]
Debt Instrument [Line Items]
Senior notes	0.0		$ 400.0
Effective Interest Rate			2.97%
Debt Face Amount	$ 400.0
Stated Interest Rate	2.85%
Notes Due October 2030 [Member] \| Franklin Resources, Inc. [Member]
Debt Instrument [Line Items]
Senior notes	$ 847.9		$ 847.5
Effective Interest Rate	1.74%		1.74%
Debt Face Amount	$ 850.0
Stated Interest Rate	1.60%
Notes Due August 2051 [Member] \| Franklin Resources, Inc. [Member]
Debt Instrument [Line Items]
Senior notes	$ 348.1		$ 348.0
Effective Interest Rate	3.00%		3.00%
Debt Face Amount	$ 350.0
Stated Interest Rate	2.95%
Notes Due July 2024 [Member] \| Legg Mason [Member]
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Debt Face Amount		$ 400.0
Stated Interest Rate		2.85%
Notes Due March 2026 [Member] \| Legg Mason [Member]
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Senior notes	$ 456.1		$ 469.5
Effective Interest Rate	1.80%		1.80%
Debt Face Amount	$ 450.0
Stated Interest Rate	4.75%
Notes Due January 2044 [Member] \| Legg Mason [Member]
Debt Instrument [Line Items]
Senior notes	$ 717.4		$ 723.9
Effective Interest Rate	3.38%		3.38%
Debt Face Amount	$ 550.0
Stated Interest Rate	5.625%