Significant Accounting Policies Significant Accounting Policies - Effects of the changes to the previously reported statement of income (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019	Jul. 31, 2020	Sep. 30, 2018
Receivables	$ 1,428.2	$ 1,233.1
Investments	1,510.3	1,304.5
Cash and cash equivalents	4,647.2	3,989.8	$ 6,206.6		$ 6,910.6
Total investments, at fair value	588.3	504.8
Total Assets	24,168.4	21,684.5
Accounts payable and accrued expenses	479.3	426.9
Debt	3,399.4	3,017.1		$ 2,000.0
Total liabilities	11,424.8	10,273.5
Nonredeemable noncontrolling interests	587.2	754.6
Total stockholders’ equity	11,810.6	10,869.1	10,541.5		$ 10,207.9
Total Liabilities, Redeemable Noncontrolling Interests and Stockholders’ Equity	24,168.4	21,684.5
Net Cash Provided by (Used in) Operating Activities	1,245.4	1,083.3	268.5
Net Cash Provided by (Used in) Investing Activities	(2,615.9)	(4,061.9)	(1,275.4)
Net Cash Provided by (Used in) Financing Activities	2,030.1	734.4	339.9
Consolidated Investment Products [Member]
Receivables	127.8	118.3
Cash and cash equivalents	289.4	963.0
Total investments, at fair value	5,820.1	4,074.0
Total Assets	6,237.3	5,155.3
Accounts payable and accrued expenses	558.0	611.2
Debt	3,671.0	2,800.6
Total liabilities	4,242.8	3,423.9
Nonredeemable noncontrolling interests	371.3	579.7
Total stockholders’ equity	1,372.0	1,334.1
Total Liabilities, Redeemable Noncontrolling Interests and Stockholders’ Equity	$ 6,237.3	5,155.3
Previously Reported [Member]
Receivables		1,200.6
Investments		1,270.5
Total Assets		20,220.9
Total liabilities		8,705.2
Nonredeemable noncontrolling interests		859.3
Total stockholders’ equity		10,973.8
Total Liabilities, Redeemable Noncontrolling Interests and Stockholders’ Equity		20,220.9
Net Cash Provided by (Used in) Operating Activities		1,021.4	201.6
Net Cash Provided by (Used in) Investing Activities		(3,243.1)	(1,077.1)
Net Cash Provided by (Used in) Financing Activities		194.2	(40.5)
Previously Reported [Member] | Consolidated Investment Products [Member]
Cash and cash equivalents		930.7
Total investments, at fair value		2,709.2
Accounts payable and accrued expenses		510.1
Debt		1,333.4
Restatement Adjustment [Member]
Receivables		32.5
Investments		34.0
Total Assets		1,463.6
Total liabilities		1,568.3
Nonredeemable noncontrolling interests		(104.7)
Total stockholders’ equity		(104.7)
Total Liabilities, Redeemable Noncontrolling Interests and Stockholders’ Equity		1,463.6
Net Cash Provided by (Used in) Operating Activities		61.9	66.9
Net Cash Provided by (Used in) Investing Activities		(818.8)	(198.3)
Net Cash Provided by (Used in) Financing Activities		540.2	380.4
Restatement Adjustment [Member] | Consolidated Investment Products [Member]
Cash and cash equivalents		32.3
Total investments, at fair value		1,364.8
Accounts payable and accrued expenses		101.1
Debt		1,467.2
As Amended [Member]
Receivables		1,233.1
Investments		1,304.5
Total Assets		21,684.5
Total liabilities		10,273.5
Nonredeemable noncontrolling interests		754.6
Total stockholders’ equity		10,869.1
Total Liabilities, Redeemable Noncontrolling Interests and Stockholders’ Equity		21,684.5
Net Cash Provided by (Used in) Operating Activities		1,083.3	268.5
Net Cash Provided by (Used in) Investing Activities		(4,061.9)	(1,275.4)
Net Cash Provided by (Used in) Financing Activities		734.4	$ 339.9
As Amended [Member] | Consolidated Investment Products [Member]
Cash and cash equivalents		963.0
Total investments, at fair value		4,074.0
Accounts payable and accrued expenses		611.2
Debt		$ 2,800.6