Acquisition - Allocation of Estimated Fair Values (Details) - USD ($) $ in Millions		Sep. 30, 2021	Sep. 30, 2020	Jul. 31, 2020	Sep. 30, 2019
Business Acquisition [Line Items]
Goodwill		$ 4,457.7	$ 4,500.8		$ 2,130.3
Initial Estimated Fair Value
Business Acquisition [Line Items]
Cash and cash equivalents				$ 681.1
Investments				471.8
Receivables				525.7
Indefinite-lived intangible assets				2,727.8
Definite-lived intangibles assets	[1]			1,353.8
Goodwill				2,325.0
Deferred tax assets				148.4
Other assets				530.7
Debt				(2,324.4)
Compensation and benefits				(579.9)
Deferred tax liabilities				(315.4)
Other liabilities				(926.4)
Total Identifiable Net Assets				4,737.2
Adjustments [Member]
Business Acquisition [Line Items]
Cash and cash equivalents				0.0
Investments				0.0
Receivables				0.0
Indefinite-lived intangible assets				0.0
Definite-lived intangibles assets	[1]			0.0
Goodwill				(52.4)
Deferred tax assets				57.6
Other assets				0.0
Debt				0.0
Compensation and benefits				0.0
Deferred tax liabilities				0.9
Other liabilities				(6.1)
Total Identifiable Net Assets				0.0
Revised Estimated Fair Value
Business Acquisition [Line Items]
Cash and cash equivalents				681.1
Investments				471.8
Receivables				525.7
Indefinite-lived intangible assets				2,727.8
Definite-lived intangibles assets	[1]			1,353.8
Goodwill				2,272.6
Deferred tax assets				206.0
Other assets				530.7
Debt				(2,324.4)
Compensation and benefits				(579.9)
Deferred tax liabilities				(314.5)
Other liabilities				(932.5)
Total Identifiable Net Assets				4,737.2
Consolidated Investment Products [Member] | Initial Estimated Fair Value
Business Acquisition [Line Items]
Cash and cash equivalents				253.4
Investments				402.9
Debt				(330.8)
Consolidated Investment Products [Member] | Adjustments [Member]
Business Acquisition [Line Items]
Cash and cash equivalents				0.0
Investments				0.0
Debt				0.0
Consolidated Investment Products [Member] | Revised Estimated Fair Value
Business Acquisition [Line Items]
Cash and cash equivalents				253.4
Investments				402.9
Debt				(330.8)
Redeemable Noncontrolling Interest [Member] | Initial Estimated Fair Value
Business Acquisition [Line Items]
Noncontrolling Interest				(186.4)
Redeemable Noncontrolling Interest [Member] | Adjustments [Member]
Business Acquisition [Line Items]
Noncontrolling Interest				0.0
Redeemable Noncontrolling Interest [Member] | Revised Estimated Fair Value
Business Acquisition [Line Items]
Noncontrolling Interest				(186.4)
Nonredeemable Noncontrolling Interest [Member] | Initial Estimated Fair Value
Business Acquisition [Line Items]
Noncontrolling Interest				(20.1)
Nonredeemable Noncontrolling Interest [Member] | Adjustments [Member]
Business Acquisition [Line Items]
Noncontrolling Interest				0.0
Nonredeemable Noncontrolling Interest [Member] | Revised Estimated Fair Value
Business Acquisition [Line Items]
Noncontrolling Interest				$ (20.1)

[1] Includes $1,123.2 million related to management contracts and $230.6 million related to trade names.