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	Mar. 31, 2017	Dec. 31, 2016
Level I [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	$ 339,508	$ 549,351
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	10,200
Closely-held Equity Security	0	0
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]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	0	0
Securities Purchased Under Agreements to Resell	535	12,585
Accounts Receivable	201,013	230,522
Receivable from Employees and Related Parties	18,063	15,034
Assets Segregated for Bank Regulatory Requirements	0	0
Closely-held Equity Security	0	0
Accounts Payable and Accrued Expenses	29,649	30,723
Securities Sold Under Agreements to Repurchase	35,368	31,150
Payable to Employees and Related Parties	36,461	27,366
Notes Payable	168,237	170,251
Subordinated Borrowings	10,835	16,803
Level III [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	0	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	0	0
Closely-held Equity Security	1,079	1,079
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]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	339,508	549,351
Securities Purchased Under Agreements to Resell	535	12,585
Accounts Receivable	201,013	230,522
Receivable from Employees and Related Parties	18,063	15,034
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Closely-held Equity Security	1,079	1,079
Accounts Payable and Accrued Expenses	29,649	30,723
Securities Sold Under Agreements to Repurchase	35,368	31,150
Payable to Employees and Related Parties	36,461	27,366
Notes Payable	168,158	168,097
Subordinated Borrowings	10,550	16,550
Estimated Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	339,508	549,351
Securities Purchased Under Agreements to Resell	535	12,585
Accounts Receivable	201,013	230,522
Receivable from Employees and Related Parties	18,063	15,034
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Closely-held Equity Security	1,079	1,079
Accounts Payable and Accrued Expenses	29,649	30,723
Securities Sold Under Agreements to Repurchase	35,368	31,150
Payable to Employees and Related Parties	36,461	27,366
Notes Payable	168,237	170,251
Subordinated Borrowings	$ 10,835	$ 16,803