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Fair Value Measurements of Assets and Liabilities - Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
Assets    
Marketable securities $ 102,839  
Fair value measurements, recurring [Member]    
Assets    
Marketable securities 102,839 $ 100,543
Total 103,085 102,152
Fair value measurements, recurring [Member] | Money market mutual funds [Member]    
Assets    
Cash equivalents 246 1,609
Fair value measurements, recurring [Member] | Commercial paper [Member]    
Assets    
Marketable securities 7,485 2,499
Fair value measurements, recurring [Member] | Corporate notes and bonds [Member]    
Assets    
Marketable securities 48,859 57,105
Fair value measurements, recurring [Member] | Government obligations [Member]    
Assets    
Marketable securities 13,991 1,996
Fair value measurements, recurring [Member] | U.S. government and agency [Member]    
Assets    
Marketable securities 32,504 38,943
Fair value measurements, recurring [Member] | Level 1 [Member]    
Assets    
Total 246 1,609
Fair value measurements, recurring [Member] | Level 1 [Member] | Money market mutual funds [Member]    
Assets    
Cash equivalents 246 1,609
Fair value measurements, recurring [Member] | Level 2 [Member]    
Assets    
Marketable securities 102,839 100,543
Total 102,839 100,543
Fair value measurements, recurring [Member] | Level 2 [Member] | Commercial paper [Member]    
Assets    
Marketable securities 7,485 2,499
Fair value measurements, recurring [Member] | Level 2 [Member] | Corporate notes and bonds [Member]    
Assets    
Marketable securities 48,859 57,105
Fair value measurements, recurring [Member] | Level 2 [Member] | Government obligations [Member]    
Assets    
Marketable securities 13,991 1,996
Fair value measurements, recurring [Member] | Level 2 [Member] | U.S. government and agency [Member]    
Assets    
Marketable securities $ 32,504 $ 38,943