Earnings (Loss) per Share - Components of Basic and Diluted Earnings (Loss) per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Earnings Per Share Reconciliation [Abstract]
Net income (loss) attributable to Franklin Resources, Inc.	$ (583.3)	$ 440.2
Less: allocation of earnings to participating nonvested stock and stock unit awards - basic	1.0	3.0
Less: allocation of earnings to participating nonvested stock and stock unit awards - diluted	1.0	3.0
Net Income (Loss) Available to Common Stockholders - basic	(584.3)	437.2
Net Income (Loss) Available to Common Stockholders - diluted	$ (584.3)	$ 437.2
Weighted-average shares outstanding – basic	550.7	565.1
Dilutive effect of nonparticipating nonvested stock unit awards	0.0	0.1
Weighted-Average Shares Outstanding – Diluted	550.7	565.2
Earnings (Loss) per Share [Abstract]
Basic	$ (1.06)	$ 0.77
Diluted	$ (1.06)	$ 0.77