Fair Value Measurements (Schedule Of Categorization Of Investments And Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	$ 199,841		$ 254,757
Cash and Cash Equivalents	144,547		136,614
Corporate Bonds, Municipal Bonds And Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	38,445	[1]	106,951	[1]
Seed Capital Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	11,106		11,690
Mutual Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	11,081		8,938
Financial Instruments Owned And Pledged As Collateral At Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	139,209		127,178
Municipal Bonds And Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents	17,794		46,291
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	159,184		145,266
Cash and Cash Equivalents	144,547		136,614
Fair Value, Inputs, Level 1 [Member] | Seed Capital Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	8,894		9,150
Fair Value, Inputs, Level 1 [Member] | Mutual Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	11,081		8,938
Fair Value, Inputs, Level 1 [Member] | Financial Instruments Owned And Pledged As Collateral At Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	139,209		127,178
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	40,657		109,491
Fair Value, Inputs, Level 2 [Member] | Corporate Bonds, Municipal Bonds And Other Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	38,445	[1]	106,951	[1]
Fair Value, Inputs, Level 2 [Member] | Seed Capital Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	$ 2,212		$ 2,540

[1] Includes $17,794 and $46,291 of municipal bonds and commercial paper classified within Cash and Cash Equivalents on the Unaudited Condensed Consolidated Statements of Financial Condition as of September 30, 2012 and December 31, 2011, respectively.