Revenues - Schedule of Operating Revenues by Geographic Area (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	$ 8,425.5	$ 5,566.5	$ 5,669.4
Investment management fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	6,541.6	3,981.7	3,985.2
Sales and distribution fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	1,635.5	1,362.0	1,444.6
Shareholder servicing fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	211.2	195.1	216.3
Other [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	37.2	27.7	23.3
United States [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	5,979.0	3,594.8	3,396.2
United States [Member] | Investment management fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	4,647.7	2,482.5	2,260.6
United States [Member] | Sales and distribution fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	1,137.4	928.8	941.3
United States [Member] | Shareholder servicing fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	164.7	158.6	175.7
United States [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	29.2	24.9	18.6
Luxembourg [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	1,507.9	1,302.9	1,533.5
Luxembourg [Member] | Investment management fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	1,075.0	910.1	1,064.7
Luxembourg [Member] | Sales and distribution fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	395.8	366.1	437.2
Luxembourg [Member] | Shareholder servicing fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	36.1	25.5	30.1
Luxembourg [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	1.0	1.2	1.5
Americas Excluding United States [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	338.3	321.4	388.8
Americas Excluding United States [Member] | Investment management fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	285.6	269.2	325.4
Americas Excluding United States [Member] | Sales and distribution fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	52.5	51.9	63.3
Americas Excluding United States [Member] | Shareholder servicing fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	0.2	0.3	0.1
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	387.9	240.2	254.5
Asia-Pacific [Member] | Investment management fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	333.3	217.6	241.8
Asia-Pacific [Member] | Sales and distribution fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	46.1	13.6	1.3
Asia-Pacific [Member] | Shareholder servicing fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	6.6	8.4	10.4
Asia-Pacific [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	1.9	0.6	1.0
Europe, Middle East and Africa, Excluding Luxembourg [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	212.4	107.2	96.4
Europe, Middle East and Africa, Excluding Luxembourg [Member] | Investment management fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	200.0	102.3	92.7
Europe, Middle East and Africa, Excluding Luxembourg [Member] | Sales and distribution fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	3.7	1.6	1.5
Europe, Middle East and Africa, Excluding Luxembourg [Member] | Shareholder servicing fees [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	3.6	2.3	0.0
Europe, Middle East and Africa, Excluding Luxembourg [Member] | Other [Member]
Disaggregation of Revenue [Line Items]
Earned From Contracts With Customers	$ 5.1	$ 1.0	$ 2.2