Investments of Operating Entities and Consolidated Funds - Operating Entities - Carried Interest (Details) - USD ($) $ in Thousands		Mar. 31, 2020		Dec. 31, 2019
Investment Holdings [Line Items]
Other investments		$ 163,274		$ 185,722
Carried interest [Member]
Investment Holdings [Line Items]
Other investments		20,729	[1]	30,360
Cowen Healthcare Investments II LP | Carried interest [Member] | Affiliated Entity
Investment Holdings [Line Items]
Other investments	[2]	14,798		23,759
Other Private Investment | Carried interest [Member] | Affiliated Entity
Investment Holdings [Line Items]
Other investments	[2],[3]	3,737		4,737
RCG IO Renergys Sarl | Carried interest [Member] | Affiliated Entity
Investment Holdings [Line Items]
Other investments	[2]	1,740		1,251
Ramius Mutli-Strategy Fund LP [Member] | Carried interest [Member] | Affiliated Entity
Investment Holdings [Line Items]
Other investments	[2]	$ 454		$ 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] These carried interest balances are earned from affiliates of the Company.

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