Acquisitions - Purchase Price Allocation (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended		0 Months Ended	3 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Jun. 28, 2011 LaBranche and Co Inc. [Member]	Jun. 30, 2011 LaBranche and Co Inc. [Member]
Business Acquisition [Line Items]
Cash and cash equivalents					$ 117,496
Cash collateral pledged					1,127
Securities owned, at fair value					221,855
Other investments					2,569
Receivable from brokers					93,754
Fixed assets, net					8,804
Intangibles					2,770
Other assets					5,137
Securities sold, not yet purchased, at fair value					(175,391)
Payable to brokers					(81,536)
Compensation payable					(3,521)
Fees payable					(969)
Unfavorable lease					(3,388)
Accounts payable, accrued expenses and other liabilities					(12,725)
Total net assets acquired					175,982
Non compete agreements and covenants with limiting conditions acquired					2,310
Goodwill/(Bargain purchase gain) on transaction	0	0	0	(22,244)	(22,244)	(22,244)
Total purchase price					$ 156,048