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	Jun. 30, 2015	Dec. 31, 2014
Level I [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	$ 240,175	$ 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
Cost Method Investments, Fair Value Disclosure	0
Loans Receivable, Fair Value Disclosure	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	33,270	7,669
Accounts Receivable	124,671	136,280
Receivable from Employees and Related Parties	17,174	17,327
Assets Segregated for Bank Regulatory Requirements	0	0
Cost Method Investments, Fair Value Disclosure	0
Loans Receivable, Fair Value Disclosure	3,500
Accounts Payable and Accrued Expenses	35,676	37,104
Securities Sold Under Agreements to Repurchase	79,784	106,499
Payable to Employees and Related Parties	31,681	18,875
Notes Payable	128,543	131,340
Subordinated Borrowings	23,167	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
Cost Method Investments, Fair Value Disclosure	1,079
Loans Receivable, Fair Value Disclosure	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	240,175	342,908
Securities Purchased Under Agreements to Resell	33,270	7,669
Accounts Receivable	124,671	136,280
Receivable from Employees and Related Parties	17,174	17,327
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Cost Method Investments, Fair Value Disclosure	1,079
Loans Receivable, Fair Value Disclosure	3,500
Accounts Payable and Accrued Expenses	35,676	37,104
Securities Sold Under Agreements to Repurchase	79,784	106,499
Payable to Employees and Related Parties	31,681	18,875
Notes Payable	128,543	131,340
Subordinated Borrowings	23,167	22,550
Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and Cash Equivalents	240,175	342,908
Securities Purchased Under Agreements to Resell	33,270	7,669
Accounts Receivable	124,671	136,280
Receivable from Employees and Related Parties	17,174	17,327
Assets Segregated for Bank Regulatory Requirements	10,200	10,200
Cost Method Investments, Fair Value Disclosure	1,079
Loans Receivable, Fair Value Disclosure	3,500
Accounts Payable and Accrued Expenses	35,676	37,104
Securities Sold Under Agreements to Repurchase	79,784	106,499
Payable to Employees and Related Parties	31,681	18,875
Notes Payable	106,285	105,226
Subordinated Borrowings	$ 22,550	$ 22,550