Segment Operating Results - (Footnotes) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Other Revenue, net	$ 5,855	$ 1,140	$ 7,206	$ (1,628)
Net Realized and Unrealized Gains on Private Equity Fund Investments	(337)	1,208	(195)	(985)
Special Charges	1,967	0	3,864	21,507
Acquisition and Transition Costs	0	599	21	976
Total Other Expenses	7,888	12,240	21,826	34,639
Investment Banking [Member]
Segment Reporting Information [Line Items]
Other Revenue, net	4,108	(878)	3,219	(3,431)
Interest expense on Notes Payable, Subordinated Borrowing and Line of Credit	2,300	2,488	6,861	7,494
Amortization of LP Units / Interests and Certain Other Awards	3,764	9,249	11,470	4,980
Special Charges	1,967	0	3,864	14,400
Acquisition and Transition Costs	0	107	0	107
Intangible Asset and Other Amortization	2,157	2,392	6,471	7,176
Total Other Expenses	7,888	11,748	21,805	26,663
Investment Management [Member]
Segment Reporting Information [Line Items]
Other Revenue, net	1,747	2,018	3,987	1,803
Special Charges	0	0	0	7,107
Acquisition and Transition Costs	0	492	21	869
Total Other Expenses	$ 0	492	$ 21	7,976
Other Revenue, Including Interest and Investments [Member]
Segment Reporting Information [Line Items]
Principal Trading Losses		(343)		(606)
Net Realized and Unrealized Gains on Private Equity Fund Investments		1,952		1,427
Other Revenue, Including Interest and Investments [Member] | Investment Banking [Member]
Segment Reporting Information [Line Items]
Principal Trading Losses		(343)		(606)
Other Revenue, Including Interest and Investments [Member] | Investment Management [Member]
Segment Reporting Information [Line Items]
Net Realized and Unrealized Gains on Private Equity Fund Investments		$ 1,952		$ 1,427