Quarterly report pursuant to Section 13 or 15(d)

Financial Instruments - Financial Instruments Measured on Recurring Basis (Detail)

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Financial Instruments - Financial Instruments Measured on Recurring Basis (Detail) (USD $)
Sep. 30, 2013
Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value, available-for-sale securities $ 60,189,000 $ 60,089,000
Money market funds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value, available-for-sale securities 3,670,000 4,643,000
Fair Value Measurements Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value, available-for-sale securities 60,189,000 60,089,000
Fair Value Measurements Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | US Government Agencies Debt Securities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value, available-for-sale securities 19,780,000 6,003,000
Fair Value Measurements Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Corporate debt securities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value, available-for-sale securities 36,739,000 49,443,000
Fair Value Measurements Recurring [Member] | Estimate of Fair Value, Fair Value Disclosure [Member] | Money market funds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value, available-for-sale securities 3,670,000 4,643,000
Fair Value Measurements Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value, available-for-sale securities 3,670,000 4,643,000
Fair Value Measurements Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | US Government Agencies Debt Securities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value, available-for-sale securities 0 0
Fair Value Measurements Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Corporate debt securities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value, available-for-sale securities 0 0
Fair Value Measurements Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Money market funds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value, available-for-sale securities 3,670,000 4,643,000
Fair Value Measurements Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value, available-for-sale securities 56,519,000 55,446,000
Fair Value Measurements Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | US Government Agencies Debt Securities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value, available-for-sale securities 19,780,000 6,003,000
Fair Value Measurements Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Corporate debt securities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value, available-for-sale securities 36,739,000 49,443,000
Fair Value Measurements Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member] | Money market funds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value, available-for-sale securities 0 0
Fair Value Measurements Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value, available-for-sale securities 0 0
Fair Value Measurements Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | US Government Agencies Debt Securities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value, available-for-sale securities 0 0
Fair Value Measurements Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Corporate debt securities [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value, available-for-sale securities 0 0
Fair Value Measurements Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member] | Money market funds [Member]
   
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Fair value, available-for-sale securities $ 0 $ 0