Consolidated Investment Products - Schedule of Financial Instruments of CIPs not Measured at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2021		Sep. 30, 2020	Jul. 31, 2020	Sep. 30, 2019	Sep. 30, 2018
Financial Assets [Abstract]
Cash and cash equivalents	$ 4,647.2		$ 3,989.8		$ 6,206.6	$ 6,910.6
Financial Liability [Abstract]
Debt	3,399.4		3,017.1	$ 2,000.0
CIPs [Member]
Financial Assets [Abstract]
Cash and cash equivalents	289.4		963.0
Financial Liability [Abstract]
Debt	3,671.0		2,800.6
Carrying Value [Member]
Financial Assets [Abstract]
Cash and cash equivalents	4,357.8		3,026.8
Carrying Value [Member] | CIPs [Member]
Financial Assets [Abstract]
Cash and cash equivalents	144.0		435.7
Financial Liability [Abstract]
Debt	36.9		153.7
Estimated Fair Value [Member] | Level 1 [Member]
Financial Assets [Abstract]
Cash and cash equivalents	4,357.8		3,026.8
Estimated Fair Value [Member] | CIPs [Member] | Level 1 [Member]
Financial Assets [Abstract]
Cash and cash equivalents	144.0		435.7
Estimated Fair Value [Member] | CIPs [Member] | Level 3 [Member]
Financial Liability [Abstract]
Debt	36.6		155.2
Collateralized Loan Obligations [Member]
Financial Liability [Abstract]
Debt	3,629.9		2,803.1
Collateralized Loan Obligations [Member] | CIPs [Member]
Financial Liability [Abstract]
Debt	3,634.1		2,646.9
Collateralized Loan Obligations [Member] | Carrying Value [Member] | CIPs [Member]
Financial Liability [Abstract]
Debt	3,634.1		2,646.9
Collateralized Loan Obligations [Member] | Estimated Fair Value [Member] | CIPs [Member] | Level 2 [Member]
Financial Liability [Abstract]
Debt	$ 3,610.6	[1]	$ 2,683.2

[1] Substantially all was Level 2.