Revenue - Schedule of Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Management fee revenue	$ 178.5	$ 207.5
Quant & Solutions \| Global / non-U.S. equity
Disaggregation of Revenue [Line Items]
Management fee revenue	85.2	90.5
Alternatives \| Alternatives
Disaggregation of Revenue [Line Items]
Management fee revenue	41.4	43.8
Liquid Alpha \| Global / non-U.S. equity
Disaggregation of Revenue [Line Items]
Management fee revenue	18.8	26.4
Liquid Alpha \| Fixed income
Disaggregation of Revenue [Line Items]
Management fee revenue	6.3	6.4
Liquid Alpha \| U.S. equity
Disaggregation of Revenue [Line Items]
Management fee revenue	$ 26.8	$ 40.4

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130543-203045

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130545-203045

Name:	us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration