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	Jun. 30, 2017	Dec. 31, 2016
Level I [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	$ 394,442	$ 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	19,471	12,585
Accounts Receivable	180,723	230,522
Receivable from Employees and Related Parties	15,950	15,034
Assets Segregated for Bank Regulatory Requirements	0	0
Closely-held Equity Security	0	0
Accounts Payable and Accrued Expenses	30,486	30,723
Securities Sold Under Agreements to Repurchase	36,543	31,150
Payable to Employees and Related Parties	38,434	27,366
Notes Payable	172,198	170,251
Subordinated Borrowings	6,887	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	394,442	549,351
Securities Purchased Under Agreements to Resell	19,471	12,585
Accounts Receivable	180,723	230,522
Receivable from Employees and Related Parties	15,950	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	30,486	30,723
Securities Sold Under Agreements to Repurchase	36,543	31,150
Payable to Employees and Related Parties	38,434	27,366
Notes Payable	168,220	168,097
Subordinated Borrowings	6,799	16,550
Estimated Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	394,442	549,351
Securities Purchased Under Agreements to Resell	19,471	12,585
Accounts Receivable	180,723	230,522
Receivable from Employees and Related Parties	15,950	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	30,486	30,723
Securities Sold Under Agreements to Repurchase	36,543	31,150
Payable to Employees and Related Parties	38,434	27,366
Notes Payable	172,198	170,251
Subordinated Borrowings	$ 6,887	$ 16,803