Concentrations of Credit Risk (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Concentration Risk [Line Items]
Securities Purchased Under Agreements to Resell	$ 10,645	$ 12,585
Securities Sold Under Agreements to Repurchase	30,027	31,150
Accounts Receivable, Net of Allowance	$ 184,993	230,522
Investment Banking and Investment Management Receivables Collection Periods	90 days
Collection Period for Restructuring Transactions and Private Equity Fee Receivables	90 days
Bad Debt Expense	$ 2,579	2,261	$ 1,314
Marketable Securities	$ 65,032	66,487
Percentage of Marketable Securities Related to Corporate and Municipal Bonds and Other Debt Securities	55.00%
Percentage of Marketable Securities Related to Equity Securities, Exchange Traded Funds and Mutual Funds	45.00%
Asset (Liability) Balance [Member]
Concentration Risk [Line Items]
Securities Purchased Under Agreements to Resell	$ 10,645	12,585
Securities Sold Under Agreements to Repurchase	30,027	31,150
Market Value of Collateral Received or (Pledged) [Member]
Concentration Risk [Line Items]
Securities Purchased Under Agreements to Resell	10,643	12,601
Securities Sold Under Agreements to Repurchase	$ 30,020	$ 31,155