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Cash Cash Equivalents and Marketable Securities - Additional Information (Detail)
 9 Months Ended
Sep. 30, 2015
USD ($)
Investment
Sep. 30, 2014
USD ($)
Cash, cash equivalents and marketable securities [Line Items]    
Long-term marketable securities, minimum original maturity term 1 year  
Marketable securities, average remaining contractual maturity period 5 months 15 days  
Number of debt securities in unrealized loss positions | Investment 14  
Debt securities in unrealized loss positions, total unrealized loss $ 1,000  
Debt securities in unrealized loss positions, total fair market value 4,920,000  
Credit risk 0  
Gain (loss) on investments $ 0 $ 0
Minimum    
Cash, cash equivalents and marketable securities [Line Items]    
Marketable securities, minimum original maturity term 90 days