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Fair Value Measurements Categorization of Investments and Certain Other Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Mar. 31, 2016
Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Assets Measured At Fair Value $ 87,064 $ 95,182
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 44,600 44,144
Securities Investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Assets Measured At Fair Value 6,877 6,949
Mutual Funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Assets Measured At Fair Value 2,339 2,347
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 33,248 41,742
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 6,951 9,653
Level I [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Assets Measured At Fair Value 40,708 49,289
Cash and Cash Equivalents 284,572 439,111
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 5,121 5,200
Level I [Member] | Mutual Funds    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Assets Measured At Fair Value 2,339 2,347
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 33,248 41,742
Level II [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Assets Measured At Fair Value 46,356 45,893
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 44,600 44,144
Level II [Member] | Securities Investments    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Total Assets Measured At Fair Value 1,756 1,749
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