Fair Value Measurements 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, 2014		Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	$ 145,925		$ 113,972
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	34,343	[1]	33,882	[1]
Securities Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	7,192	[1]	14,399	[1]
Mutual Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	5,702		9,380
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	98,688		56,311
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	9,252		14,254
Level I [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	109,940		77,692
Cash and Cash Equivalents	342,908		284,199
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 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	5,550	[1]	12,001	[1]
Level I [Member] | Mutual Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	5,702		9,380
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	98,688		56,311
Level II [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	35,985		36,280
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	34,343	[1]	33,882	[1]
Level II [Member] | Securities Investments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total Assets Measured At Fair Value	1,642		2,398	[1]
Level II [Member] | Mutual Funds [Member]
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 [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] | Mutual Funds [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] | 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 $9,252 and $14,254 of treasury bills, municipal bonds and commercial paper classified within Cash and Cash Equivalents on the Consolidated Statements of Financial Condition as of December 31, 2014 and 2013, respectively.