Goodwill and Intangibles Schedule of Goodwill by Segment (Details) (Annual) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Line Items]
Goodwill, gross	$ 161,513	$ 80,363	$ 80,363
Accumulated impairment charges	(23,785)	(19,685)	(19,685)
Goodwill, net	(137,728)	(60,678)	(60,678)
Recognized goodwill	81,150	0
Realignment, goodwill	0
Realignment goodwill, accumulated impairment charges	0
Goodwill Impairment	(4,100)	0	0
Investment Management Reporting Unit
Goodwill [Line Items]
Goodwill, gross	0	29,026	29,026
Accumulated impairment charges	0	(10,200)	(10,200)
Goodwill, net	0	(18,826)	18,826
Recognized goodwill	0	0
Realignment, goodwill	(29,026)
Realignment goodwill, accumulated impairment charges	10,200
Goodwill Impairment	0	0
Investment Bank Reporting Unit
Goodwill [Line Items]
Goodwill, gross	132,487	51,337	51,337
Accumulated impairment charges	(9,485)	(9,485)	(9,485)
Goodwill, net	(123,002)	(41,852)	(41,852)
Recognized goodwill	81,150	0
Realignment, goodwill	0
Realignment goodwill, accumulated impairment charges	0
Goodwill Impairment	0	0
Cowen Investment Management Reporting Unit
Goodwill [Line Items]
Goodwill, gross	22,705	0	0
Accumulated impairment charges	(7,979)	0	0
Goodwill, net	(14,726)	0	0
Recognized goodwill	0
Realignment, goodwill	22,705
Realignment goodwill, accumulated impairment charges	(7,979)
Goodwill Impairment	0
Asset Company Reporting Unit
Goodwill [Line Items]
Goodwill, gross	6,321	0	0
Accumulated impairment charges	(6,321)	0	0
Goodwill, net	0	$ 0	$ 0
Recognized goodwill	0
Realignment, goodwill	6,321
Realignment goodwill, accumulated impairment charges	(2,221)
Goodwill Impairment	(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
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

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