Fair Value Measurements Carrying Amount and Estimated Fair Value of Financial Instrument Assets and Liabilities which are Not Measured at Fair Value (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Level I [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	$ 439,111	$ 342,908
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
Loans Receivable	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	2,191	7,669
Accounts Receivable	175,497	136,280
Receivable from Employees and Related Parties	21,189	17,327
Assets Segregated for Bank Regulatory Requirements	0	0
Closely-held Equity Security	0
Loans Receivable	3,666
Accounts Payable and Accrued Expenses	43,878	37,104
Securities Sold Under Agreements to Repurchase	44,000	106,499
Payable to Employees and Related Parties	28,392	18,875
Notes Payable	120,373	131,340
Subordinated Borrowings	23,076	22,550
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
Loans Receivable	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
Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	439,111	342,908
Securities Purchased Under Agreements to Resell	2,191	7,669
Accounts Receivable	175,497	136,280
Receivable from Employees and Related Parties	21,189	17,327
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Closely-held Equity Security	1,079
Loans Receivable	3,666
Accounts Payable and Accrued Expenses	43,878	37,104
Securities Sold Under Agreements to Repurchase	44,000	106,499
Payable to Employees and Related Parties	28,392	18,875
Notes Payable	120,373	131,340
Subordinated Borrowings	23,076	22,550
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	439,111	342,908
Securities Purchased Under Agreements to Resell	2,191	7,669
Accounts Receivable	175,497	136,280
Receivable from Employees and Related Parties	21,189	17,327
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Closely-held Equity Security	1,079
Loans Receivable	3,500
Accounts Payable and Accrued Expenses	43,878	37,104
Securities Sold Under Agreements to Repurchase	44,000	106,499
Payable to Employees and Related Parties	28,392	18,875
Notes Payable	119,250	105,226
Subordinated Borrowings	$ 22,550	$ 22,550