Revenues - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2018
Revenue from Contract with Customer [Abstract]
Subadvisory fees for mutual funds	$ 11.4	$ 36.6

Subadvisory fees that the funds adviser pays for out of the management fee, which it receives from the fund.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.14)
-URI http://asc.fasb.org/extlink&oid=114873765&loc=SL114874048-224260

Name:	us-gaap_SubadvisoryFeesForMutualFunds
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration