Goodwill Qtlry (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, gross	$ 164,548	$ 164,548	$ 161,513
Accumulated impairment charges	(17,464)	(17,464)	(23,785)
Goodwill	147,084	147,084	137,728
Goodwill Impairment	0	0
Recognized goodwill	0	9,356
Investment Bank Reporting Unit
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, gross	141,843	141,843	132,487
Accumulated impairment charges	(9,485)	(9,485)	(9,485)
Goodwill	132,358	132,358	123,002
Goodwill Impairment	0	0
Recognized goodwill	0	9,356
Cowen Investment Management Reporting Unit
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, gross	22,705	22,705	22,705
Accumulated impairment charges	(7,979)	(7,979)	(7,979)
Goodwill	14,726	14,726	14,726
Goodwill Impairment	0	0
Recognized goodwill	0	0
Asset Company Reporting Unit
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, gross	0	0	6,321
Accumulated impairment charges	0	0	(6,321)
Goodwill	0	0	$ 0
Goodwill Impairment	0	0
Recognized goodwill	$ 0	$ 0