Investments of Operating Entities and Consolidated Funds - Operating Entities - Carried Interest (Details) - USD ($) $ in Thousands		Sep. 30, 2021	Dec. 31, 2020
Investment Holdings [Line Items]
Other investments		$ 262,431	$ 255,027
Carried interest
Investment Holdings [Line Items]
Other investments	[1]	84,312	82,892
Cowen Healthcare Investments III LP | Carried interest | Affiliated Entity
Investment Holdings [Line Items]
Other investments		31,223	62,112
Cowen Healthcare Investments III LP | Carried interest | Affiliated Entity
Investment Holdings [Line Items]
Other investments		10,889	11,520
Cowen Sustainable Investments I LP | Carried interest | Affiliated Entity
Investment Holdings [Line Items]
Other investments		6,122	0
Cowen Sustainable Investments Offshore I LP | Carried interest | Affiliated Entity
Investment Holdings [Line Items]
Other investments		7,505	0
CSI I Prodigy Co-Investment LP | Carried interest | Affiliated Entity
Investment Holdings [Line Items]
Other investments		2,817	0
CSI PRTA Co-Investment LP | Carried interest | Affiliated Entity
Investment Holdings [Line Items]
Other investments		11,073	0
Triartisan TGIF Partners LLC | Carried interest | Affiliated Entity
Investment Holdings [Line Items]
Other investments		4,311	3,361
Triartisan ES Partners, LLC | Carried interest | Affiliated Entity
Investment Holdings [Line Items]
Other investments		3,416	3,152
Triartisan PFC Partners, LLC | Carried interest | Affiliated Entity
Investment Holdings [Line Items]
Other investments		5,803	1,455
Ramius Multi-Strategy Fund LP | Carried interest | Affiliated Entity
Investment Holdings [Line Items]
Other investments		615	734
Ramius Merger Fund LLC | Carried interest | Affiliated Entity
Investment Holdings [Line Items]
Other investments		374	368
RCG IO Renergys Sarl | Carried interest | Affiliated Entity
Investment Holdings [Line Items]
Other investments		$ 164	$ 190

[1] In accordance with US GAAP, portfolio funds are measured at fair value using the net asset value per share (or its equivalent) as a practical expedient are not classified in the fair value hierarchy. Carried interest and equity method investments presented in the table above are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the condensed consolidated statement of financial condition.