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Fair Value Hierarchy for Financial Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) (Fair Value, Measurements, Recurring, USD $)
In Thousands, unless otherwise specified
Dec. 31, 2012
Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets $ 123,588 $ 111,671
Money market funds
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets 3,140 17,171
US Government Agencies Debt Securities
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets 119,233 28,495
US Treasury Securities
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets 500 7,425
Municipal Securities
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets 715  
Corporate Debt Securities, Secured by the U.S. Government
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets   58,580
Fair Value, Inputs, Level 1
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets 3,140 17,171
Fair Value, Inputs, Level 1 | Money market funds
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets 3,140 17,171
Fair Value, Inputs, Level 2
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets 120,448 94,500
Fair Value, Inputs, Level 2 | US Government Agencies Debt Securities
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets 119,233 28,495
Fair Value, Inputs, Level 2 | US Treasury Securities
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets 500 7,425
Fair Value, Inputs, Level 2 | Municipal Securities
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets 715  
Fair Value, Inputs, Level 2 | Corporate Debt Securities, Secured by the U.S. Government
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets   $ 58,580