v3.24.1.u1
Basis of Presentation - Effects of the changes to the previously reported statements of income and cash flows (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Mar. 31, 2024
Mar. 31, 2023
Statement [Line Items]        
Net cash used in operating activities     $ (115.3) $ (329.6)
Net cash used in investing activities     (410.8) (2,221.3)
Net cash provided by financing activities     232.1 1,986.8
Decrease to cash and cash equivalents     (278.1) (503.9)
Investment and other income, net $ 52.5 $ 60.6 225.7 150.5
Other income, net 108.8 110.9 233.5 144.8
Income before taxes 238.1 366.0 569.3 593.9
Net Income 175.3 273.1 431.6 440.7
Nonredeemable noncontrolling interests 8.3 (4.3) 3.8 (0.8)
Net income attributable to Franklin Resources, Inc. 124.2 194.2 375.5 359.8
Operating Income $ 129.3 255.1 $ 335.8 449.1
Previously Reported [Member]        
Statement [Line Items]        
Net cash used in operating activities       (289.7)
Net cash used in investing activities       (2,195.0)
Net cash provided by financing activities       1,920.6
Decrease to cash and cash equivalents       (503.9)
Investment and other income, net   125.6   216.7
Other income, net   175.9   211.0
Income before taxes   431.0   660.1
Net Income   338.1   506.9
Nonredeemable noncontrolling interests   60.7   65.4
Net income attributable to Franklin Resources, Inc.   194.2   359.8
Operating Income   255.1   449.1
Revision of Prior Period, Adjustment [Member]        
Statement [Line Items]        
Net cash used in operating activities       (39.9)
Net cash used in investing activities       (26.3)
Net cash provided by financing activities       66.2
Decrease to cash and cash equivalents       0.0
Investment and other income, net   (65.0)   (66.2)
Other income, net   (65.0)   (66.2)
Income before taxes   (65.0)   (66.2)
Net Income   (65.0)   (66.2)
Nonredeemable noncontrolling interests   (65.0)   (66.2)
Net income attributable to Franklin Resources, Inc.   0.0   0.0
Operating Income   0.0   0.0
As Amended [Member]        
Statement [Line Items]        
Net cash used in operating activities       (329.6)
Net cash used in investing activities       (2,221.3)
Net cash provided by financing activities       1,986.8
Decrease to cash and cash equivalents       (503.9)
Investment and other income, net   60.6   150.5
Other income, net   110.9   144.8
Income before taxes   366.0   593.9
Net Income   273.1   440.7
Nonredeemable noncontrolling interests   (4.3)   (0.8)
Net income attributable to Franklin Resources, Inc.   194.2   359.8
Operating Income   $ 255.1   $ 449.1