Consolidated Investment Products - Schedule of Financial Instruments of CIPs not Measured at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Financial Assets [Abstract]
Cash and cash equivalents	$ 3,957.5	$ 5,957.6	$ 6,910.6	$ 8,749.7
Financial Liability [Abstract]
Debt	3,017.1	696.9
CIPs [Member]
Financial Assets [Abstract]
Cash and cash equivalents	930.7	154.2
Financial Liability [Abstract]
Debt	1,333.4	50.8
Carrying Value [Member]
Financial Assets [Abstract]
Cash and cash equivalents	3,026.8	5,803.4
Carrying Value [Member] | CIPs [Member]
Financial Assets [Abstract]
Cash and cash equivalents	441.9	154.2
Financial Liability [Abstract]
Debt	153.7	50.8
Estimated Fair Value [Member] | Level 1 [Member]
Financial Assets [Abstract]
Cash and cash equivalents	3,026.8	5,803.4
Estimated Fair Value [Member] | CIPs [Member] | Level 1 [Member]
Financial Assets [Abstract]
Cash and cash equivalents	441.9	154.2
Estimated Fair Value [Member] | CIPs [Member] | Level 3 [Member]
Financial Liability [Abstract]
Debt	155.2	51.0
Collateralized Loan Obligations [Member]
Financial Liability [Abstract]
Debt	1,235.8
Collateralized Loan Obligations [Member] | CIPs [Member]
Financial Liability [Abstract]
Debt	1,179.7	0.0
Collateralized Loan Obligations [Member] | Carrying Value [Member] | CIPs [Member]
Financial Liability [Abstract]
Debt	1,179.7	0.0
Collateralized Loan Obligations [Member] | Estimated Fair Value [Member] | CIPs [Member] | Level 2 [Member]
Financial Liability [Abstract]
Debt	$ 1,225.0	$ 0.0