Acquisitions Purchase Price Allocation (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended				0 Months Ended	3 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 ATM Group and Cowen Equity Finance LP [Member]	Jun. 28, 2011 LaBranche and Co Inc. [Member]	Jun. 30, 2011 LaBranche and Co Inc. [Member]
Business Acquisition [Line Items]
Cash and cash equivalents				$ 290	$ 117,496
Cash collateral pledged					1,127
Securities owned, at fair value				17	221,855
Securities Borrowed	408,096	0		527,854
Other investments					2,569
Receivable from brokers				15,682	93,754
Fees receivable, net of allowance	34,707	28,315		751
Fixed assets, net					8,804
Intangibles				9,782
Other assets				136	5,137
Securities Loaned	(410,441)	0		(543,369)
Securities sold, not yet purchased, at fair value					(175,391)
Payable to brokers					(81,536)
Compensation payable				(11)	(3,521)
Fees payable				(56)	(969)
Unfavorable lease				(91)	(3,388)
Accounts payable, accrued expenses and other liabilities				(700)	(12,725)
Total net assets acquired				10,285	175,982
Non compete agreements and covenants with limiting conditions acquired recognized separately				167	2,310
Goodwill/(Bargain purchase gain) on transaction	0	(22,244)	0	(8,517)	(22,244)	(22,200)
Total purchase price				$ 18,969	$ 156,048