Revenue from Contracts with Customers - Disaggregation of Revenue by Asset Class (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Total Revenue	$ 340,340	$ 308,616	$ 968,869	$ 828,461
Equity
Disaggregation of Revenue [Line Items]
Total Revenue	136,655	138,624	394,324	344,323
Money Market
Disaggregation of Revenue [Line Items]
Total Revenue	138,242	101,257	378,686	304,662
Fixed-Income
Disaggregation of Revenue [Line Items]
Total Revenue	45,018	45,897	133,476	135,442
Other
Disaggregation of Revenue [Line Items]
Total Revenue	$ 20,425	$ 22,838	$ 62,383	$ 44,034

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