Investments of Operating Entities and Consolidated Funds - Operating Entities - Carried Interest (Details) - USD ($) $ in Thousands		Sep. 30, 2020		Dec. 31, 2019
Investment Holdings [Line Items]
Other investments		$ 204,610		$ 185,722
Carried interest
Investment Holdings [Line Items]
Other investments		47,343	[1]	30,360
Cowen Healthcare Investments II LP | Carried interest | Affiliated Entity
Investment Holdings [Line Items]
Other investments		35,829		23,759
Cowen Healthcare Investments III LP | Carried interest | Affiliated Entity
Investment Holdings [Line Items]
Other investments		5,357		0
Triartisan TGIF Partners LLC | Carried interest | Affiliated Entity
Investment Holdings [Line Items]
Other investments		2,881		0
Triartisan ES Partners, LLC | Carried interest | Affiliated Entity
Investment Holdings [Line Items]
Other investments		1,354		0
Triartisan PFC Partners, LLC | Carried interest | Affiliated Entity
Investment Holdings [Line Items]
Other investments		1,116		0
Ramius Multi-Strategy Fund LP | Carried interest | Affiliated Entity
Investment Holdings [Line Items]
Other investments		612		0
Merger Fund | Carried interest | Affiliated Entity
Investment Holdings [Line Items]
Other investments		13		0
Other Private Investment | Carried interest | Affiliated Entity
Investment Holdings [Line Items]
Other investments	[2]	0		4,737
RCG IO Renergys Sarl | Carried interest | Affiliated Entity
Investment Holdings [Line Items]
Other investments		181		1,251
Ramius Mutli-Strategy Fund LP | Carried interest | Affiliated Entity
Investment Holdings [Line Items]
Other investments		$ 0		$ 613

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

[2] Other private investment represents the Company's closed end investment in a Portfolio Fund that invests in a wireless broadband communication provider in Italy.