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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
Sep. 30, 2013
Dec. 31, 2012
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]    
Assets $ 74,381 $ 123,588
Money market funds
    
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]    
Assets 9,598 3,140
US Government Agencies Debt Securities
    
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]    
Assets 62,283 119,233
US Treasury Securities
    
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]    
Assets 2,500 500
Municipal Securities
    
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]    
Assets   715
Fair Value, Inputs, Level 1
    
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]    
Assets 9,598 3,140
Fair Value, Inputs, Level 1 | Money market funds
    
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]    
Assets 9,598 3,140
Fair Value, Inputs, Level 2
    
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]    
Assets 64,783 120,448
Fair Value, Inputs, Level 2 | US Government Agencies Debt Securities
    
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]    
Assets 62,283 119,233
Fair Value, Inputs, Level 2 | US Treasury Securities
    
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]    
Assets 2,500 500
Fair Value, Inputs, Level 2 | Municipal Securities
    
Fair Value Assets And Liabilities Measured On Recurring Basis [Line Items]    
Assets   $ 715