Goodwill and Intangibles Schedule of Goodwill by Segment (Details) (Annual) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Goodwill [Line Items]
Goodwill, gross	$ 161,513,000		$ 161,513,000		$ 80,363,000	$ 80,363,000
Accumulated impairment charges	(23,785,000)		(23,785,000)		(19,685,000)	(19,685,000)
Goodwill, net	(137,728,000)		(137,728,000)		(60,678,000)	(60,678,000)
Recognized goodwill			81,150,000		0
Realignment, goodwill	0		0
Realignment goodwill, accumulated impairment charges	0		0
Goodwill Impairment	0	$ 0	(4,100,000)	$ 0	0
Investment Management Reporting Unit
Goodwill [Line Items]
Goodwill, gross	0		0		29,026,000	29,026,000
Accumulated impairment charges	0		0		(10,200,000)	(10,200,000)
Goodwill, net	0		0		(18,826,000)	18,826,000
Recognized goodwill			0		0
Realignment, goodwill	(29,026,000)		(29,026,000)
Realignment goodwill, accumulated impairment charges	10,200,000		10,200,000
Goodwill Impairment			0		0
Investment Bank Reporting Unit
Goodwill [Line Items]
Goodwill, gross	132,487,000		132,487,000		51,337,000	51,337,000
Accumulated impairment charges	(9,485,000)		(9,485,000)		(9,485,000)	(9,485,000)
Goodwill, net	(123,002,000)		(123,002,000)		(41,852,000)	(41,852,000)
Recognized goodwill			81,150,000		0
Realignment, goodwill	0		0
Realignment goodwill, accumulated impairment charges	0		0
Goodwill Impairment			0		0
Cowen Investment Management Reporting Unit
Goodwill [Line Items]
Goodwill, gross	22,705,000		22,705,000		0	0
Accumulated impairment charges	(7,979,000)		(7,979,000)		0	0
Goodwill, net	(14,726,000)		(14,726,000)		0	0
Recognized goodwill			0
Realignment, goodwill	22,705,000		22,705,000
Realignment goodwill, accumulated impairment charges	(7,979,000)		(7,979,000)
Goodwill Impairment			0
Asset Company Reporting Unit
Goodwill [Line Items]
Goodwill, gross	6,321,000		6,321,000		0	0
Accumulated impairment charges	(6,321,000)		(6,321,000)		0	0
Goodwill, net	0		0		$ 0	$ 0
Recognized goodwill			0
Realignment, goodwill	6,321,000		6,321,000
Realignment goodwill, accumulated impairment charges	$ (2,221,000)		(2,221,000)
Goodwill Impairment			$ (4,100,000)