Revenues - Schedule of Operating Revenues by Geographic Area (Details) - USD ($) $ in Millions	12 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2023
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	$ 8,770.7	$ 8,478.0	$ 7,849.4
Investment management fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	6,981.8	6,822.2	6,452.9
Sales and distribution fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	1,474.7	1,381.0	1,203.7
Shareholder servicing fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	264.5	229.3	152.7
Other [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	49.7	45.5	40.1
United States [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	6,614.2	6,357.8	5,880.8
United States [Member] \| Investment management fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	5,292.3	5,142.8	4,877.1
United States [Member] \| Sales and distribution fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	1,042.2	979.2	847.3
United States [Member] \| Shareholder servicing fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	231.3	195.3	118.7
United States [Member] \| Other [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	48.4	40.5	37.7
Luxembourg [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	1,295.7	1,237.5	1,132.2
Luxembourg [Member] \| Investment management fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	894.4	862.3	803.9
Luxembourg [Member] \| Sales and distribution fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	370.0	342.8	296.0
Luxembourg [Member] \| Shareholder servicing fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	31.2	31.7	31.5
Luxembourg [Member] \| Other [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	0.1	0.7	0.8
Americas Excluding United States [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	254.0	268.0	257.1
Americas Excluding United States [Member] \| Investment management fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	214.8	228.1	216.2
Americas Excluding United States [Member] \| Sales and distribution fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	38.9	39.8	40.6
Americas Excluding United States [Member] \| Shareholder servicing fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	0.3	0.1	0.3
Americas Excluding United States [Member] \| Other [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	0.0	0.0	0.0
Asia-Pacific [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	327.8	308.9	308.8
Asia-Pacific [Member] \| Investment management fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	302.3	283.8	285.6
Asia-Pacific [Member] \| Sales and distribution fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	22.7	19.2	19.8
Asia-Pacific [Member] \| Shareholder servicing fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	1.7	2.2	2.2
Asia-Pacific [Member] \| Other [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	1.1	3.7	1.2
Europe, Middle East and Africa, Excluding Luxembourg [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	279.0	305.8	270.5
Europe, Middle East and Africa, Excluding Luxembourg [Member] \| Investment management fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	278.0	305.2	270.1
Europe, Middle East and Africa, Excluding Luxembourg [Member] \| Sales and distribution fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	0.9	0.0	0.0
Europe, Middle East and Africa, Excluding Luxembourg [Member] \| Shareholder servicing fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	0.0	0.0	0.0
Europe, Middle East and Africa, Excluding Luxembourg [Member] \| Other [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	$ 0.1	$ 0.6	$ 0.4