Earnings per Share - Components of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Earnings Per Share Reconciliation [Abstract]
Net income attributable to Franklin Resources, Inc.	$ 174.0	$ 227.5	$ 549.5	$ 587.3
Less: allocation of earnings to participating nonvested stock and stock unit awards - basic	7.5	10.2	23.8	26.7
Less: allocation of earnings to participating nonvested stock and stock unit awards - diluted	7.5	10.2	23.8	26.7
Net Income Available to Common Stockholders - basic	166.5	217.3	525.7	560.6
Net Income Available to Common Stockholders - diluted	$ 166.5	$ 217.3	$ 525.7	$ 560.6
Weighted-average shares outstanding – basic	516.5	490.7	507.2	490.3
Dilutive effect of nonparticipating nonvested stock unit awards	0.7	0.7	0.8	0.7
Weighted-Average Shares Outstanding – Diluted	517.2	491.4	508.0	491.0
Earnings per Share [Abstract]
Basic	$ 0.32	$ 0.44	$ 1.04	$ 1.14
Diluted	$ 0.32	$ 0.44	$ 1.03	$ 1.14