Acquisitions Purchase Price Allocation (Details) (USD $)	12 Months Ended					3 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 ATM Group and Cowen Equity Finance LP [Member]	Dec. 31, 2013 ATM Group and Cowen Equity Finance LP [Member]	Jun. 30, 2011 LaBranche and Co Inc. [Member]	Jun. 28, 2011 LaBranche and Co Inc. [Member]
Business Acquisition [Line Items]
Cash and cash equivalents				$ 290,000
Securities owned, at fair value				17,000
Securities borrowed				527,854,000
Receivable from brokers				15,682,000
Fees receivable				751,000
Intangibles				9,782,000
Other assets				136,000
Securities loaned				(543,369,000)
Compensation payable				(11,000)
Fees payable				(56,000)
Unfavorable lease liability				(91,000)
Accounts payable, accrued expenses and other liabilities				(700,000)
Total net assets acquired				10,285,000			176,000,000
Non compete agreements				167,000			2,300,000
Goodwill/(Bargain purchase gain) on transaction	0	0	(22,244,000)	(8,517,000)		(22,200,000)
Total purchase price				18,969,000
Intangible assets					$ 9,900,000		$ 5,100,000