Divestitures and Assets Held for Sale - Assets and liabilities of affiliate held for sale (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Investments, at fair value	$ 139.9	$ 0.0
Consolidated Entity Excluding Consolidated Funds
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Investments, at fair value	27.0	0.0
Affiliate assets held for sale	151.0	0.0
Affiliate liabilities held for sale	80.4	0.0
Consolidated Funds
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Investments, at fair value	112.9	0.0
Affiliate assets held for sale	114.3	0.0
Affiliate liabilities held for sale	0.1	$ 0.0
Disposal Group, Held-for-sale, Not Discontinued Operations | Barrow Hanley | Consolidated Entity Excluding Consolidated Funds
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash and cash equivalents	12.2
Investment advisory fees receivable	21.5
Investments, at fair value	27.0
Goodwill	76.1
Right of use assets	12.4
Other Assets	1.8
Affiliate assets held for sale	151.0
Accrued incentive compensation	8.3
Other compensation liabilities	56.4
Operating lease liabilities	12.7
Accounts payable and accrued expenses	3.0
Affiliate liabilities held for sale	80.4
Disposal Group, Held-for-sale, Not Discontinued Operations | Barrow Hanley | Consolidated Funds
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Cash and cash equivalents	0.8
Investments, at fair value	112.9
Other Assets	0.6
Affiliate assets held for sale	114.3
Accounts payable and accrued expenses	0.1
Affiliate liabilities held for sale	$ 0.1