Fair Value Measurements - Carrying Amount and Estimated Fair Value of Financial Instrument Assets and Liabilities which are Not Measured at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Level I [Member]
Financial Assets:
Cash and Cash Equivalents	$ 596,141	$ 549,351
Certificates of Deposit	0
Debt Security Investment	0
Securities Purchased Under Agreements to Resell	0	0
Accounts Receivable	0	0
Receivable from Employees and Related Parties	0	0
Assets Segregated for Bank Regulatory Requirements		10,200
Closely-held Equity Security	0	0
Financial Liabilities:
Accounts Payable and Accrued Expenses	0	0
Securities Sold Under Agreements to Repurchase	0	0
Payable to Employees and Related Parties	0	0
Notes Payable	0	0
Subordinated Borrowings	0	0
Level II [Member]
Financial Assets:
Cash and Cash Equivalents	0	0
Certificates of Deposit	63,527
Debt Security Investment	0
Securities Purchased Under Agreements to Resell	10,645	12,585
Accounts Receivable	184,993	230,522
Receivable from Employees and Related Parties	17,030	15,034
Assets Segregated for Bank Regulatory Requirements		0
Closely-held Equity Security	0	0
Financial Liabilities:
Accounts Payable and Accrued Expenses	34,111	30,723
Securities Sold Under Agreements to Repurchase	30,027	31,150
Payable to Employees and Related Parties	31,167	27,366
Notes Payable	171,929	170,251
Subordinated Borrowings	6,859	16,803
Level III [Member]
Financial Assets:
Cash and Cash Equivalents	0	0
Certificates of Deposit	0
Debt Security Investment	10,995
Securities Purchased Under Agreements to Resell	0	0
Accounts Receivable	0	0
Receivable from Employees and Related Parties	0	0
Assets Segregated for Bank Regulatory Requirements		0
Closely-held Equity Security	1,079	1,079
Financial Liabilities:
Accounts Payable and Accrued Expenses	0	0
Securities Sold Under Agreements to Repurchase	0	0
Payable to Employees and Related Parties	0	0
Notes Payable	0	0
Subordinated Borrowings	0	0
Carrying Amount [Member]
Financial Assets:
Cash and Cash Equivalents	596,141	549,351
Certificates of Deposit	63,527
Debt Security Investment	10,995
Securities Purchased Under Agreements to Resell	10,645	12,585
Accounts Receivable	184,993	230,522
Receivable from Employees and Related Parties	17,030	15,034
Assets Segregated for Bank Regulatory Requirements		10,200
Closely-held Equity Security	1,079	1,079
Financial Liabilities:
Accounts Payable and Accrued Expenses	34,111	30,723
Securities Sold Under Agreements to Repurchase	30,027	31,150
Payable to Employees and Related Parties	31,167	27,366
Notes Payable	168,347	168,097
Subordinated Borrowings	6,799	16,550
Total [Member]
Financial Assets:
Cash and Cash Equivalents	596,141	549,351
Certificates of Deposit	63,527
Debt Security Investment	10,995
Securities Purchased Under Agreements to Resell	10,645	12,585
Accounts Receivable	184,993	230,522
Receivable from Employees and Related Parties	17,030	15,034
Assets Segregated for Bank Regulatory Requirements		10,200
Closely-held Equity Security	1,079	1,079
Financial Liabilities:
Accounts Payable and Accrued Expenses	34,111	30,723
Securities Sold Under Agreements to Repurchase	30,027	31,150
Payable to Employees and Related Parties	31,167	27,366
Notes Payable	171,929	170,251
Subordinated Borrowings	$ 6,859	$ 16,803