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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
Mar. 31, 2013
Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets $ 102,806 $ 123,588
Money market funds
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets 18,783 3,140
US Government Agencies Debt Securities
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets 80,058 119,233
US Treasury Securities
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets 3,250 500
Municipal Securities
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets 715 715
Fair Value, Inputs, Level 1
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets 18,783 3,140
Fair Value, Inputs, Level 1 | Money market funds
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets 18,783 3,140
Fair Value, Inputs, Level 2
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets 84,023 120,448
Fair Value, Inputs, Level 2 | US Government Agencies Debt Securities
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets 80,058 119,233
Fair Value, Inputs, Level 2 | US Treasury Securities
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets 3,250 500
Fair Value, Inputs, Level 2 | Municipal Securities
    
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]    
Assets $ 715 $ 715