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Financial Instruments - Summary of Cash Equivalents and Short and Long-term Investments (Detail) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Financial Instruments [Member]    
Schedule of Financial Instruments [Line Items]    
Due within one year $ 153,802 $ 144,736
Amortized Cost 153,648 149,953
Due within two years   5,206
Unrealized Gains (Losses) 154 (11)
Fair Value 153,802 149,942
Money Market Funds [Member]    
Schedule of Financial Instruments [Line Items]    
Amortized Cost 109,265 104,544
Fair Value 109,265 104,544
Certificate of Deposit [Member]    
Schedule of Financial Instruments [Line Items]    
Amortized Cost 7,000 7,814
Fair Value 7,000 7,814
Corporate Bonds [Member]    
Schedule of Financial Instruments [Line Items]    
Amortized Cost 15,194 17,758
Unrealized Gains (Losses) 40 (57)
Fair Value 15,234 17,701
Commercial Paper [Member]    
Schedule of Financial Instruments [Line Items]    
Amortized Cost 9,889 7,456
Unrealized Gains (Losses) 76 20
Fair Value 9,965 7,476
U.S. Treasury Securities [Member]    
Schedule of Financial Instruments [Line Items]    
Amortized Cost 12,300 12,381
Unrealized Gains (Losses) 38 26
Fair Value $ 12,338 $ 12,407