Investments of Operating Entities and Consolidated Funds - Operating Entities - Gross Obligations For Securities Loaned And Securities Sold Under Agreements To Repurchase (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Common stock	$ 1,326,917	$ 1,601,866
Securities sold, not yet purchased, at fair value	440,930	451,836
Common stock
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Common stock	933,956	1,343,478
Common stock | Open and Overnight
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Common stock	933,956	1,343,478
Common stock | Up to 30 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Common stock	0	0
Common stock | 31 - 90 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Common stock	0	0
Common stock | Greater than 90 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Common stock	0	0
Corporate Bonds
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Common stock	392,961	258,388
Securities sold, not yet purchased, at fair value	6,218	23,244
Corporate Bonds | Open and Overnight
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Common stock	392,961	258,388
Securities sold, not yet purchased, at fair value	0	0
Corporate Bonds | Up to 30 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Common stock	0	0
Securities sold, not yet purchased, at fair value	0	0
Corporate Bonds | 31 - 90 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Common stock	0	0
Securities sold, not yet purchased, at fair value	6,218	23,244
Corporate Bonds | Greater than 90 days
Transfer of Certain Financial Assets Accounted for as Secured Borrowings [Line Items]
Common stock	0	0
Securities sold, not yet purchased, at fair value	$ 0	$ 0