Goodwill and Intangibles Schedule of Goodwill by Segment (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Alternative Investment [Member]	Dec. 31, 2011 Alternative Investment [Member]	Dec. 31, 2012 Broker-Dealer [Member]	Dec. 31, 2011 Broker-Dealer [Member]	Dec. 31, 2009 Prior transactions [Member] Alternative Investment [Member]	Dec. 31, 2009 Prior transactions [Member] Broker-Dealer [Member]	Dec. 31, 2012 ATM Group and Cowen Equity Finance LP [Member] Broker-Dealer [Member]
Goodwill [Roll Forward]
Goodwill, Gross	$ 37,379	$ 37,379		$ 30,228	$ 30,228	$ 7,151	$ 7,151
Accumulated Impairment Charges	(17,351)	(10,200)		(10,200)	(10,200)	(7,151)	0
Goodwill, Net	20,028	27,179		20,028	20,028	0	7,151
Recognized goodwill	8,517	0		0	0	8,517	0	30,200	7,200	8,500
Goodwill impairment	0	(7,151)	0	0	0	0	(7,151)
Goodwill, Gross	45,896	37,379	37,379	30,228	30,228	15,668	7,151
Accumulated Impairment Charges	(17,351)	(17,351)	(10,200)	(10,200)	(10,200)	(7,151)	(7,151)
Goodwill, Net	$ 28,545	$ 20,028	$ 27,179	$ 20,028	$ 20,028	$ 8,517	$ 0