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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share Reconciliation [Abstract]
Net income attributable to Franklin Resources, Inc.	$ 245.9	$ 402.0	$ 889.3	$ 261.9
Less: allocation of earnings to participating nonvested stock and stock unit awards - basic	2.6	3.3	8.8	17.1
Less: allocation of earnings to participating nonvested stock and stock unit awards - diluted	2.6	3.3	8.8	17.1
Net Income Available to Common Stockholders - basic	243.3	398.7	880.5	244.8
Net Income Available to Common Stockholders - diluted	$ 243.3	$ 398.7	$ 880.5	$ 244.8
Weighted-average shares outstanding – basic	501.6	533.0	505.6	542.9
Dilutive effect of nonparticipating nonvested stock unit awards	0.7	0.5	0.5	0.7
Weighted-Average Shares Outstanding – Diluted	502.3	533.5	506.1	543.6
Earnings per Share [Abstract]
Basic	$ 0.48	$ 0.75	$ 1.74	$ 0.45
Diluted	$ 0.48	$ 0.75	$ 1.74	$ 0.45