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	Sep. 30, 2017	Dec. 31, 2016
Level I [Member]
Financial Assets:
Cash and Cash Equivalents	$ 414,918	$ 549,351
Certificates of Deposit	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	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	63,417
Securities Purchased Under Agreements to Resell	11,268	12,585
Accounts Receivable	206,885	230,522
Receivable from Employees and Related Parties	16,934	15,034
Assets Segregated for Bank Regulatory Requirements	0	0
Closely-held Equity Security	0	0
Financial Liabilities:
Accounts Payable and Accrued Expenses	33,001	30,723
Securities Sold Under Agreements to Repurchase	33,912	31,150
Payable to Employees and Related Parties	27,832	27,366
Notes Payable	170,284	170,251
Subordinated Borrowings	6,968	16,803
Level III [Member]
Financial Assets:
Cash and Cash Equivalents	0	0
Certificates of Deposit	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
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	414,918	549,351
Certificates of Deposit	63,417
Securities Purchased Under Agreements to Resell	11,268	12,585
Accounts Receivable	206,885	230,522
Receivable from Employees and Related Parties	16,934	15,034
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Closely-held Equity Security	1,079	1,079
Financial Liabilities:
Accounts Payable and Accrued Expenses	33,001	30,723
Securities Sold Under Agreements to Repurchase	33,912	31,150
Payable to Employees and Related Parties	27,832	27,366
Notes Payable	168,282	168,097
Subordinated Borrowings	6,799	16,550
Estimated Fair Value [Member]
Financial Assets:
Cash and Cash Equivalents	414,918	549,351
Certificates of Deposit	63,417
Securities Purchased Under Agreements to Resell	11,268	12,585
Accounts Receivable	206,885	230,522
Receivable from Employees and Related Parties	16,934	15,034
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Closely-held Equity Security	1,079	1,079
Financial Liabilities:
Accounts Payable and Accrued Expenses	33,001	30,723
Securities Sold Under Agreements to Repurchase	33,912	31,150
Payable to Employees and Related Parties	27,832	27,366
Notes Payable	170,284	170,251
Subordinated Borrowings	$ 6,968	$ 16,803