Significant Accounting Policies All Other - Quarterly (Details) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($) cowenfund	Dec. 31, 2016 USD ($)
Total net assets of consolidated VIEs	$ 475.5	$ 461.6
Deferred rent	$ 13.5	$ 10.3
Other investment companies
Number of funds, Consolidated | cowenfund	6
Assets Held under Capital Leases
Property, Plant and Equipment, Useful Life	5 years
Minimum | Telephone and computer equipment
Property, Plant and Equipment, Useful Life	3 years
Minimum | Computer Software, Intangible Asset
Property, Plant and Equipment, Useful Life	3 years
Minimum | Furniture and Fixtures
Property, Plant and Equipment, Useful Life	5 years
Minimum | Leasehold improvements
Property, Plant and Equipment, Useful Life	2 years
Minimum | Aircraft
Property, Plant and Equipment, Useful Life	10 years
Minimum | Flight Equipment
Property, Plant and Equipment, Useful Life	4 years
Maximum | Telephone and computer equipment
Property, Plant and Equipment, Useful Life	5 years
Maximum | Computer Software, Intangible Asset
Property, Plant and Equipment, Useful Life	8 years
Maximum | Furniture and Fixtures
Property, Plant and Equipment, Useful Life	5 years
Maximum | Leasehold improvements
Property, Plant and Equipment, Useful Life	15 years
Maximum | Aircraft
Property, Plant and Equipment, Useful Life	20 years
Maximum | Flight Equipment
Property, Plant and Equipment, Useful Life	10 years