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Fair Value Measurements - Financial Assets Measured at Fair Value on Recurring Basis (Detail) - USD ($)
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Valuation techniques, description There were no changes to our valuation techniques during the three months ended March 31, 2016.    
Other-than-temporary impairments $ 0 $ 0  
Assets      
Marketable securities 100,961,000    
Fair value measurements, recurring [Member]      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Financial and Nonfinancial Liabilities, Fair Value Disclosure 0   $ 0
Assets      
Cash equivalents 3,321,000    
Marketable securities 100,961,000   102,839,000
Total 104,282,000   103,085,000
Fair value measurements, recurring [Member] | Commercial paper [Member]      
Assets      
Cash equivalents 2,000,000    
Fair value measurements, recurring [Member] | Money market mutual funds [Member]      
Assets      
Cash equivalents 1,321,000   246,000
Fair value measurements, recurring [Member] | Commercial paper [Member]      
Assets      
Marketable securities 7,485,000   7,485,000
Fair value measurements, recurring [Member] | Corporate notes and bonds [Member]      
Assets      
Marketable securities 46,028,000   48,859,000
Fair value measurements, recurring [Member] | Government obligations [Member]      
Assets      
Marketable securities 16,016,000   13,991,000
Fair value measurements, recurring [Member] | U.S. agency obligations [Member]      
Assets      
Marketable securities 31,432,000   32,504,000
Fair value measurements, recurring [Member] | Level 1 [Member]      
Assets      
Cash equivalents 1,321,000    
Total 1,321,000   246,000
Fair value measurements, recurring [Member] | Level 1 [Member] | Commercial paper [Member]      
Assets      
Cash equivalents 0    
Fair value measurements, recurring [Member] | Level 1 [Member] | Money market mutual funds [Member]      
Assets      
Cash equivalents 1,321,000   246,000
Fair value measurements, recurring [Member] | Level 2 [Member]      
Assets      
Cash equivalents 2,000,000    
Marketable securities 100,961,000   102,839,000
Total 102,961,000   102,839,000
Fair value measurements, recurring [Member] | Level 2 [Member] | Commercial paper [Member]      
Assets      
Cash equivalents 2,000,000    
Fair value measurements, recurring [Member] | Level 2 [Member] | Commercial paper [Member]      
Assets      
Marketable securities 7,485,000   7,485,000
Fair value measurements, recurring [Member] | Level 2 [Member] | Corporate notes and bonds [Member]      
Assets      
Marketable securities 46,028,000   48,859,000
Fair value measurements, recurring [Member] | Level 2 [Member] | Government obligations [Member]      
Assets      
Marketable securities 16,016,000   13,991,000
Fair value measurements, recurring [Member] | Level 2 [Member] | U.S. agency obligations [Member]      
Assets      
Marketable securities $ 31,432,000   $ 32,504,000