Goodwill and Intangibles Schedule of Goodwill by Segment (Details) (Annual) - USD ($)	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2019		Jun. 30, 2018	Jun. 30, 2019	Jun. 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 charges	(4,100,000)	[1],[2],[3],[4]	$ 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 charges				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 charges				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 charges				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 charges				$ (4,100,000)

[1] Economic Income (Loss) excludes income taxes and acquisition related adjustments as management does not consider these items when evaluating the performance of the segment.

[2] Economic Income (Loss) presents underwriting expenses net of investment banking revenues, expenses reimbursedfrom clients within their respective expense category. Economic Income (Loss) also records retainer fees, relating to investment banking activities, collectible during the period that would otherwise be deferred until closing for US GAAP reporting

[3] Economic Income (Loss) recognizes Company income from proprietary trading (including interest and dividends

[4] Economic Income (Loss) recognizes the Company's proportionate share of expenses, for certain real estate operating entities and the activist business, for which the investments are recorded under the equity method of accounting for investments.