Earnings per Share - Components of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share Reconciliation [Abstract]
Net income attributable to Franklin Resources, Inc.	$ 1,291.9	$ 1,831.2	$ 798.9
Less: allocation of earnings to participating nonvested stock and stock unit awards - basic	54.1	77.7	15.3
Less: allocation of earnings to participating nonvested stock and stock unit awards - diluted	54.1	77.7	15.3
Net Income Available to Common Stockholders - basic	1,237.8	1,753.5	783.6
Net Income Available to Common Stockholders - diluted	$ 1,237.8	$ 1,753.5	$ 783.6
Weighted-average shares outstanding – basic	488.7	489.9	491.9
Dilutive effect of nonparticipating nonvested stock unit awards	0.6	0.7	0.5
Weighted-Average Shares Outstanding – Diluted	489.3	490.6	492.4
Earnings per Share [Abstract]
Basic	$ 2.53	$ 3.58	$ 1.59
Diluted	$ 2.53	$ 3.57	$ 1.59