Significant Accounting Policies - Annual (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) cowenfund	Dec. 31, 2016 USD ($)
Organization [Line Items]
Total net assets of consolidated VIEs	$ 482,800	$ 461,600
Deferred rent	$ 12,822	$ 10,335
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	5 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	8 years
Furniture and Fixtures
Organization [Line Items]
Property, Plant and Equipment, Useful Life	5 years
Leasehold improvements | Minimum
Organization [Line Items]
Property, Plant and Equipment, Useful Life	2 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] | Maximum
Organization [Line Items]
Asset Management Fees, Percent Fee	2.00%
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%
Other Healthcare Royalty Partners | Minimum
Organization [Line Items]
Asset Management Fees, Percent Fee	0.20%
Other Healthcare Royalty Partners | Maximum
Organization [Line Items]
Asset Management Fees, Percent Fee	1.00%
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%
Hedge Funds [Member]
Organization [Line Items]
Incentive Fees, Percent Fee	20.00%
Other investment companies
Organization [Line Items]
Number of funds, Consolidated | cowenfund	7