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Investment Securities - Offsetting Derivatives (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Assets
Derivative Asset	$ 6.3		$ 6.3
Liabilities
Derivative liabilities	8.1		8.1
Derivative Instrument Denominated in Foreign Currency
Investment gains (losses), net	50.3	$ 4.8	(0.2)	$ 18.1
Cash flows related to investment securities
Purchases and settlements			77.5	57.5
Seeded investment products and derivatives
Derivative Instrument Denominated in Foreign Currency
Investment gains (losses), net	11.1	0.8	6.0	3.4
Third-party ownership interests in seeded investment products
Derivative Instrument Denominated in Foreign Currency
Investment gains (losses), net	29.0	2.6	(9.4)	8.2
Long Tail Alpha
Derivative Instrument Denominated in Foreign Currency
Investment gains (losses), net	4.4	0.3	5.7	0.7
Deferred Equity Plan
Derivative Instrument Denominated in Foreign Currency
Investment gains (losses), net	4.7	1.7	(2.6)	5.4
Other investments
Derivative Instrument Denominated in Foreign Currency
Investment gains (losses), net	1.1	$ (0.6)	0.1	0.4
Investment securities
Cash flows related to investment securities
Purchases and settlements			(53.7)	(58.9)
Sales, settlements and maturities			131.2	116.4
Investment securities \| Seeded investment products \| Consolidated
Cash flows related to investment securities
Purchases and settlements			(239.6)	(269.1)
Sales, settlements and maturities			2.2	$ 246.9
Derivative Instruments \| Seeded investment products \| Futures
Seeded investment products, credit protection
Notional value of derivative	248.8		248.8		$ 88.3
Derivative Instruments \| Seeded investment products \| Contracts for differences
Seeded investment products, credit protection
Notional value of derivative	1.2		1.2		15.5
Derivative Instruments \| Seeded investment products \| Interest rate swaps
Seeded investment products, credit protection
Notional value of derivative	38.5		38.5		19.4
Derivative Instruments \| Seeded investment products \| Total return swaps
Seeded investment products, credit protection
Notional value of derivative					0.1
Derivative Instruments \| Seeded investment products \| Credit default swaps
Seeded investment products, credit protection
Notional value of derivative	18.8		18.8		0.1
Derivative Instruments \| Seeded investment products \| Options
Seeded investment products, credit protection
Notional value of derivative	7.1		7.1		1.0
Derivative Instruments \| Seeded investment products \| Foreign currency forward contracts
Seeded investment products, credit protection
Notional value of derivative	$ 383.2		$ 383.2		$ 167.5