Segment Operating Results - (Footnotes) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Other Revenue, net	$ 1,171	$ (2,785)	$ 1,351	$ (2,768)
Net Realized and Unrealized Gains on Private Equity Fund Investments	(204)	(2,532)	142	(2,193)
Special Charges	0	21,507	1,897	21,507
Acquisition and Transition Costs	0	377	21	377
Total Other Expenses	5,880	41,378	13,938	22,399
Investment Banking [Member]
Segment Reporting Information [Line Items]
Other Revenue, net	539	(1,375)	(889)	(2,553)
Interest expense on Notes Payable, Subordinated Borrowing and Line of Credit	2,300	2,425	4,561	5,006
Amortization of LP Units / Interests and Certain Other Awards	3,723	17,102	7,706	(4,269)
Special Charges	0	14,400	1,897	14,400
Intangible Asset and Other Amortization	2,157	2,392	4,314	4,784
Total Other Expenses	5,880	33,894	13,917	14,915
Investment Management [Member]
Segment Reporting Information [Line Items]
Other Revenue, net	632	(1,410)	2,240	(215)
Special Charges	0	7,107	0	7,107
Acquisition and Transition Costs	0	377	21	377
Total Other Expenses	$ 0	7,484	$ 21	7,484
Other Revenue, Including Interest and Investments [Member]
Segment Reporting Information [Line Items]
Principal Trading Losses		(253)		(263)
Net Realized and Unrealized Gains on Private Equity Fund Investments		(1,589)		(525)
Other Revenue, Including Interest and Investments [Member] | Investment Banking [Member]
Segment Reporting Information [Line Items]
Principal Trading Losses		(253)		(263)
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		$ (1,589)		$ (525)