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Equity Method Investments in Affiliates - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended				9 Months Ended
	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Estimated Future Amortization Expense of Finite-Lived Intangibles
Net income	$ 127.8		$ 162.1		$ 231.2	$ 209.3
Acquired Client Relationships
Schedule of Equity Method Investments [Line Items]
Amortization of Intangible Assets					62.7
Estimated Future Amortization Expense of Finite-Lived Intangibles
Finite-lived intangible, future amortization expense in 2020	10.0				10.0
Finite-lived intangible, future amortization expense in 2021	30.0				30.0
Finite-lived intangible, future amortization expense in 2022	30.0				30.0
Finite-lived intangible, future amortization expense in 2023	30.0				30.0
Finite-lived intangible, future amortization expense in 2024	20.0				20.0
Finite-lived intangible, future amortization expense in 2025	$ 10.0				10.0
Average projected growth rate	(14.00%)
Discount rate for assets based fees	15.00%
Market participant tax rate	25.00%
Equity Method Investee
Estimated Future Amortization Expense of Finite-Lived Intangibles
Impairment expense		$ 140.0	$ 10.0	$ 415.0
Average projected growth rate period		5 years		5 years
Average projected growth rate		(2.00%)	(20.00%)	(13.00%)
Discount rate for assets based fees		11.00%	11.00%	11.00%
Discount rate for performance fees		20.00%	20.00%	20.00%
Market participant tax rate		25.00%	25.00%	25.00%
Equity Method Investee | Acquired Client Relationships
Schedule of Equity Method Investments [Line Items]
Amortization of Intangible Assets	$ 34.3		$ 42.1		110.5	104.1
Estimated Future Amortization Expense of Finite-Lived Intangibles
Finite-lived intangible, future amortization expense in 2020	40.0				40.0
Finite-lived intangible, future amortization expense in 2021	120.0				120.0
Finite-lived intangible, future amortization expense in 2022	50.0				50.0
Finite-lived intangible, future amortization expense in 2023	50.0				50.0
Finite-lived intangible, future amortization expense in 2024	50.0				50.0
Finite-lived intangible, future amortization expense in 2025	50.0				50.0
Certain Affiliates
Estimated Future Amortization Expense of Finite-Lived Intangibles
Revenues	116.2		122.7		524.5	429.8
Net income	$ 74.2		$ 75.2		$ 379.5	$ 290.9

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