Significant Accounting Policies - Annual (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) cowenfund	Dec. 31, 2015 USD ($)
Organization [Line Items]
Total net assets of consolidated VIEs	$ 461,600	$ 1,500
Deferred rent	10,335	11,979
Deferred Rent Asset, Net, Current	$ 52	$ 341
Minimum
Organization [Line Items]
Redeemable Noncontrolling Interest, Redemption Fee, Percent	1.00%
Maximum
Organization [Line Items]
Redeemable Noncontrolling Interest, Redemption Fee, Percent	5.00%
Telephone and computer equipment | Minimum
Organization [Line Items]
Property, Plant and Equipment, Useful Life	3 years
Telephone and computer equipment | Maximum
Organization [Line Items]
Property, Plant and Equipment, Useful Life	8 years
Computer Software, Intangible Asset | Minimum
Organization [Line Items]
Property, Plant and Equipment, Useful Life	3 years
Computer Software, Intangible Asset | Maximum
Organization [Line Items]
Property, Plant and Equipment, Useful Life	7 years
Furniture and Fixtures | Minimum
Organization [Line Items]
Property, Plant and Equipment, Useful Life	5 years
Furniture and Fixtures | Maximum
Organization [Line Items]
Property, Plant and Equipment, Useful Life	8 years
Leasehold improvements | Minimum
Organization [Line Items]
Property, Plant and Equipment, Useful Life	5 years
Leasehold improvements | Maximum
Organization [Line Items]
Property, Plant and Equipment, Useful Life	15 years
Assets Held under Capital Leases
Organization [Line Items]
Property, Plant and Equipment, Useful Life	5 years
Aircraft | Minimum
Organization [Line Items]
Property, Plant and Equipment, Useful Life	10 years
Aircraft | Maximum
Organization [Line Items]
Property, Plant and Equipment, Useful Life	20 years
Flight Equipment | Minimum
Organization [Line Items]
Property, Plant and Equipment, Useful Life	4 years
Flight Equipment | Maximum
Organization [Line Items]
Property, Plant and Equipment, Useful Life	10 years
Hedge Funds [Member]
Organization [Line Items]
Asset Management Fees, Percent Fee	2.00%
Hedge Funds [Member] | Maximum
Organization [Line Items]
Asset Management Fees, Percent Fee	2.00%
Registered Funds [Member]
Organization [Line Items]
Asset Management Fees, Percent Fee	1.50%
Real Estate Funds | Minimum
Organization [Line Items]
Asset Management Fees, Percent Fee	0.25%
Real Estate Funds | Maximum
Organization [Line Items]
Asset Management Fees, Percent Fee	1.50%
Ramius Trading Strategies Managed Futures Fund [Member]
Organization [Line Items]
Asset Management Fees, Percent Fee	0.50%
Managed mutual funds [Member]
Organization [Line Items]
Asset Management Fees, Percent Fee	20.00%
Managed mutual funds [Member] | Minimum
Organization [Line Items]
Asset Management Fees, Percent Fee	1.50%
During investment period [Member] | Healthcare Royalty Partners | Minimum
Organization [Line Items]
Asset Management Fees, Percent Fee	1.00%
During investment period [Member] | Healthcare Royalty Partners | Maximum
Organization [Line Items]
Asset Management Fees, Percent Fee	2.00%
After investment period [Member] | Healthcare Royalty Partners | Minimum
Organization [Line Items]
Asset Management Fees, Percent Fee	0.50%
After investment period [Member] | Healthcare Royalty Partners | Maximum
Organization [Line Items]
Asset Management Fees, Percent Fee	2.00%
Commodity trading advisory [Member] | Ramius Trading Strategies Managed Futures Fund [Member] | Maximum
Organization [Line Items]
Asset Management Fees, Percent Fee	0.50%
Hedge Funds [Member]
Organization [Line Items]
Incentive Fees, Percent Fee	20.00%
Other investment companies
Organization [Line Items]
Number of funds, Consolidated | cowenfund	6