Goodwill and Intangibles Schedule of Intangible Assets (Details) (Annual) - USD ($) $ in Thousands		12 Months Ended
	Jun. 01, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross		$ 63,036	$ 55,187
Intangible assets, accumulated amortization (in dollars)		(33,081)	(29,418)
Intangible Assets, Net		29,955	25,769
Amortization of Intangible Assets		6,100	5,000	$ 2,700
Impairment of Intangible Assets, Finite-lived		$ 0	0
Investment Contracts [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		5 years
Trade Names
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross		$ 10,622	9,962
Intangible assets, accumulated amortization (in dollars)		(10,555)	(9,689)
Intangible Assets, Net		67	273
Customer Relationships [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross		41,134	33,874
Intangible assets, accumulated amortization (in dollars)		(15,733)	(12,948)
Intangible Assets, Net		25,401	20,926
Customer Contracts [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross		$ 800	800
Finite-Lived Intangible Asset, Useful Life		1 year 2 months 12 days
Intangible assets, accumulated amortization (in dollars)		$ (800)	(800)
Intangible Assets, Net		0	0
Noncompete Agreements [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross		2,237	2,268
Intangible assets, accumulated amortization (in dollars)		(1,479)	(818)
Intangible Assets, Net		758	1,450
Developed Technology Rights [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross		8,243	8,283
Intangible assets, accumulated amortization (in dollars)		(4,514)	(5,163)
Intangible Assets, Net		3,729	3,120
Cowen Equity Finance [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Impairment of Intangible Assets, Finite-lived				$ 0
Convergex Group [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Assets, Gross	$ 10,270
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	7 years 8 months 12 days
Intangible assets, net	$ 10,300
Amortization of Intangible Assets		$ 1,400
Convergex Group [Member] | Customer Relationships [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	9 years
Minimum | Trade Names
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		7 months 6 days
Minimum | Customer Relationships [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		3 years
Minimum | Noncompete Agreements [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		3 years
Minimum | Developed Technology Rights [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		3 years
Minimum | Convergex Group [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	7 months 6 days
Minimum | Convergex Group [Member] | Trade Names
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	7 months 6 days
Maximum | Trade Names
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		7 years 6 months
Maximum | Customer Relationships [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		14 years
Maximum | Noncompete Agreements [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		5 years
Maximum | Developed Technology Rights [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life		10 years
Maximum | Convergex Group [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	9 years
Maximum | Convergex Group [Member] | Trade Names
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life	2 years 7 months 6 days
Broker-Dealer | CRT business
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-lived Intangible Assets Acquired			$ 5,100
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life			8 years 1 month 6 days
Broker-Dealer | Minimum | CRT business
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life			1 year
Broker-Dealer | Maximum | CRT business
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life			9 years