Significant Accounting Policies All Other - Quarterly (Details) $ in Thousands	Sep. 30, 2015 USD ($) cowenfund	Dec. 31, 2014 USD ($) cowenfund
Redeemable Noncontrolling Interest [Line Items]
Total net assets of consolidated VIEs | $	$ 1,000	$ 2,000
Deferred rent | $	$ 12,242	$ 13,142
Variable Interest Entity, Primary Beneficiary [Member]
Redeemable Noncontrolling Interest [Line Items]
Number of funds, Consolidated	3	2
Other investment companies
Redeemable Noncontrolling Interest [Line Items]
Number of funds, Consolidated	3	3
Investment Company
Redeemable Noncontrolling Interest [Line Items]
Number of funds, Consolidated	4	4