Significant Accounting Policies (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Significant Accounting Policies [Line Items]
Investment Banking And Investment Management Receivables Collection Periods				90 days
Placement Fees Receivables Collection Period				180 days
Private Funds Capital Raising Receivables Collection Period				1 year
Collection Period For Restructuring Transactions And Private Equity Fee Receivables				90 days
Receivables Reflected in Other Assets in Excess of Period				1 year
Contract Asset Recognized in Benefit in Excess of Period				1 year
Other Operating Expenses(1)	[1]	$ 6,505	$ 5,442	$ 13,775	$ 11,033
Professional Fees	[1]	20,833	9,908	36,883	26,653
Net Realized and Unrealized Gains on Private Equity Fund Investments		(204)	(2,532)	142	(2,193)
Variable Interest Entity, Primary Beneficiary [Member]
Significant Accounting Policies [Line Items]
Consolidated Assets		96,871		96,871		$ 126,078
Consolidated Liabilities		$ 68,714		$ 68,714		$ 102,487
Execution, Clearing and Custody Fees [Member]
Significant Accounting Policies [Line Items]
Other Operating Expenses(1)			3,363		6,889
Professional Fees			295		628
Other Revenue, Including Interest and Investments [Member]
Significant Accounting Policies [Line Items]
Principal Trading Losses			(253)		(263)
Net Realized and Unrealized Gains on Private Equity Fund Investments			$ (1,589)		$ (525)

[1] Certain balances in the prior period were reclassified to conform to their current presentation. See Note 2 for further information.