Goodwill and Intangibles Schedule of Goodwill by Segment (Details) (Annual) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Goodwill [Line Items]
Goodwill, gross	$ 170,869	$ 161,513	$ 80,363
Accumulated impairment charges	(23,785)	(23,785)	(19,685)
Goodwill	147,084	137,728	60,678
Recognized goodwill	9,356	81,150
Goodwill, Transfers		0
Realignment goodwill, accumulated impairment charges		0
Goodwill Impairment	0	(4,100)	0
Amortization of Intangible Assets	13,000	13,100	5,000
Investment Management Reporting Unit
Goodwill [Line Items]
Goodwill, gross	0	0	29,026
Accumulated impairment charges	0	0	(10,200)
Goodwill	0	0	(18,826)
Recognized goodwill	0	0
Goodwill, Transfers		(29,026)
Realignment goodwill, accumulated impairment charges		10,200
Goodwill Impairment	0	0
Investment Bank Reporting Unit
Goodwill [Line Items]
Goodwill, gross	141,843	132,487	51,337
Accumulated impairment charges	(9,485)	(9,485)	(9,485)
Goodwill	132,358	123,002	41,852
Recognized goodwill	9,356	81,150
Goodwill, Transfers		0
Realignment goodwill, accumulated impairment charges		0
Goodwill Impairment	0	0
Cowen Investment Management Reporting Unit
Goodwill [Line Items]
Goodwill, gross	22,705	22,705	0
Accumulated impairment charges	(7,979)	(7,979)	0
Goodwill	14,726	14,726	0
Recognized goodwill	0	0
Goodwill, Transfers		22,705
Realignment goodwill, accumulated impairment charges		(7,979)
Goodwill Impairment	0	0
Asset Company Reporting Unit
Goodwill [Line Items]
Goodwill, gross	6,321	6,321	0
Accumulated impairment charges	(6,321)	(6,321)	0
Goodwill	0	0	$ 0
Recognized goodwill	0	0
Goodwill, Transfers		6,321
Realignment goodwill, accumulated impairment charges		(2,221)
Goodwill Impairment	0	(4,100)
Quarton
Goodwill [Line Items]
Recognized goodwill		81,200
Finite-lived Intangible Assets Acquired		$ 22,200
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life		2 years 9 months 18 days
Amortization of Intangible Assets	8,900	$ 8,900
Quarton | Minimum
Goodwill [Line Items]
Finite-Lived Intangible Asset, Useful Life		2 years
Quarton | Maximum
Goodwill [Line Items]
Finite-Lived Intangible Asset, Useful Life		4 years
MHT Partners [Member]
Goodwill [Line Items]
Recognized goodwill	9,400
Finite-lived Intangible Assets Acquired	$ 1,200
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	4 years 2 months 1 day
Amortization of Intangible Assets	$ 100
MHT Partners [Member] | Minimum
Goodwill [Line Items]
Finite-Lived Intangible Asset, Useful Life	3 years
MHT Partners [Member] | Maximum
Goodwill [Line Items]
Finite-Lived Intangible Asset, Useful Life	5 years