RELATED PARTIES - Revenue for Services for Managed Funds (Details) - Affiliated Entity - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Related Party Transactions [Line Items]
Total operating revenues	$ 1,604.6	$ 1,372.5	$ 4,696.7	$ 4,132.8
Investment management fees
Related Party Transactions [Line Items]
Total operating revenues	1,156.4	987.2	3,374.7	2,975.2
Service and distribution fees
Related Party Transactions [Line Items]
Total operating revenues	393.3	338.9	1,139.2	1,004.8
Performance fees
Related Party Transactions [Line Items]
Total operating revenues	0.0	0.0	5.8	3.4
Other
Related Party Transactions [Line Items]
Total operating revenues	$ 54.9	$ 46.4	$ 177.0	$ 149.4

Amount of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 10
-Section S99
-Paragraph 1B
-Subparagraph (SX 210.13-02(a)(4)(iv))
-URI http://asc.fasb.org/extlink&oid=124359900&loc=SL124442552-122756

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 220
-Section S99
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Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
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-Subparagraph (SX 210.13-01(a)(4)(iii)(B))
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Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 7: http://www.xbrl.org/2003/role/disclosureRef
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-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=123367319&loc=SL114868664-224227

+ Details