Fair Value Measurements Categorization of Investments and Certain Other Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands		Jun. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value		$ 88,538	$ 145,925
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	[1]	33,317	34,343
Securities Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	[1]	6,257	7,192
Mutual Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value		2,494	5,702
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		46,470	98,688
Treasury Bills, Municipal Bonds and Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and Cash Equivalents		9,904	9,252
Level I [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value		53,545	109,940
Cash and Cash Equivalents		240,175	342,908
Level I [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		0	0
Level I [Member] | Securities Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	[1]	4,581	5,550
Level I [Member] | Mutual Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value		2,494	5,702
Level I [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		46,470	98,688
Level II [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value		34,993	35,985
Cash and Cash Equivalents		0	0
Level II [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	[1]	33,317	34,343
Level II [Member] | Securities Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value		1,676	1,642	[1]
Level II [Member] | Mutual Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value		0	0
Level II [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		0	0
Level III [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value		0	0
Cash and Cash Equivalents		0	0
Level III [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		0	0
Level III [Member] | Securities Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value		0	0
Level III [Member] | Mutual Funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value		0	0
Level III [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		$ 0	$ 0

[1] Includes $9,904 and $9,252 of treasury bills, municipal bonds and commercial paper classified within Cash and Cash Equivalents on the Unaudited Condensed Consolidated Statements of Financial Condition as of June 30, 2015 and December 31, 2014, respectively.