Categorization of Investments and Certain Other Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	$ 182,002		$ 254,757
Corporate Bonds, Municipal Bonds and Other Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	38,482	[1]	106,951	[1]
Seed Capital Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	11,580		11,690
Mutual Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	11,346		8,938
Financial Instruments Owned and Pledged as Collateral at Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	120,594		127,178
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	141,078		145,266
Level 1 | Seed Capital Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	9,138		9,150
Level 1 | Mutual Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	11,346		8,938
Level 1 | Financial Instruments Owned and Pledged as Collateral at Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	120,594		127,178
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	40,924		109,491
Level 2 | Corporate Bonds, Municipal Bonds and Other Debt Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	38,482	[1]	106,951	[1]
Level 2 | Seed Capital Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	$ 2,442		$ 2,540

[1] Includes $24,863 and $46,291 of municipal bonds and commercial paper classified within Cash and Cash Equivalents on the Consolidated Statements of Financial Condition as of December 31, 2012 and 2011, respectively.