Quarterly report pursuant to Section 13 or 15(d)

Financial Instruments - Composition of Financial Instruments (Detail)

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Financial Instruments - Composition of Financial Instruments (Detail) (USD $)
Sep. 30, 2013
Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 60,170,000 $ 60,084,000
Available-for-sale Securities Gross Unrealized Gain Accumulated in Investments 26,000 9,000
Available-for-sale Securities Gross Unrealized Loss Accumulated in Investments 7,000 4,000
Fair Value 60,189,000 60,089,000
Cash and cash equivalents [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 3,670,000 4,643,000
Available-for-sale Securities Gross Unrealized Gain Accumulated in Investments 0 0
Available-for-sale Securities Gross Unrealized Loss Accumulated in Investments 0 0
Fair Value 3,670,000 4,643,000
Investments [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 56,500,000 55,441,000
Available-for-sale Securities Gross Unrealized Gain Accumulated in Investments 26,000 9,000
Available-for-sale Securities Gross Unrealized Loss Accumulated in Investments 7,000 4,000
Fair Value 56,519,000 55,446,000
US Government Agencies Debt Securities [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 19,770,000 6,000,000
Available-for-sale Securities Gross Unrealized Gain Accumulated in Investments 11,000 3,000
Available-for-sale Securities Gross Unrealized Loss Accumulated in Investments 1,000 0
Fair Value 19,780,000 6,003,000
Corporate debt securities [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 36,730,000 49,441,000
Available-for-sale Securities Gross Unrealized Gain Accumulated in Investments 15,000 6,000
Available-for-sale Securities Gross Unrealized Loss Accumulated in Investments 6,000 4,000
Fair Value 36,739,000 49,443,000
Money market funds [Member]
    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 3,670,000 4,643,000
Available-for-sale Securities Gross Unrealized Gain Accumulated in Investments 0 0
Available-for-sale Securities Gross Unrealized Loss Accumulated in Investments 0 0
Fair Value $ 3,670,000 $ 4,643,000