Goodwill and Intangibles Qtlry (Details) - 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
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, Impairment Loss	$ 4,100,000	[1],[2],[3],[4]	$ 0	$ 4,100,000	$ 0	$ 0
Goodwill, Acquired During Period				81,150,000		$ 0
Quarton
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, Acquired During Period				81,200,000
Finite-lived Intangible Assets Acquired				$ 22,200,000
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life				2 years 9 months 18 days
Amortization of Intangible Assets	$ 2,200,000			$ 4,500,000
Minimum | Quarton
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life				2 years
Maximum | Quarton
Acquired Finite-Lived Intangible Assets [Line Items]
Finite-Lived Intangible Asset, Useful Life				4 years
Asset Company Reporting Unit
Acquired Finite-Lived Intangible Assets [Line Items]
Goodwill, Impairment Loss				$ 4,100,000
Goodwill, Acquired During Period				$ 0

[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.