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Cash Equivalents and Investments - Amortized Cost and Fair Value of Cash Equivalents and Available for Sale Investments (Detail) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Cash And Cash Equivalents [Line Items]    
Amortized Cost $ 42,256 $ 125,206
Unrealized Gains 700  
Amortized Cost 190,654  
Unrealized Gains 3  
Unrealized Losses (76)  
Fair Value 190,581  
Money Market Funds [Member]    
Cash And Cash Equivalents [Line Items]    
Amortized Cost 34,771  
Unrealized Gains 0  
Unrealized Losses 0  
Fair Value 34,771  
U.S. Treasury Securities [Member]    
Cash And Cash Equivalents [Line Items]    
Amortized Cost 56,999  
Unrealized Gains 0  
Unrealized Losses (41)  
Fair Value 56,958  
Agency Bonds [Member]    
Cash And Cash Equivalents [Line Items]    
Amortized Cost 61,792  
Unrealized Gains 1  
Unrealized Losses (14)  
Fair Value 61,779  
Commercial Paper [Member]    
Cash And Cash Equivalents [Line Items]    
Amortized Cost 19,448  
Unrealized Gains 0  
Unrealized Losses (13)  
Fair Value 19,435  
Corporate Obligations [Member]    
Cash And Cash Equivalents [Line Items]    
Amortized Cost 17,644  
Unrealized Gains 2  
Unrealized Losses (8)  
Fair Value $ 17,638  
US Treasury Securities and Money Market Funds [Member] | Cash Equivalents [Member]    
Cash And Cash Equivalents [Line Items]    
Amortized Cost   111,501
Unrealized Gains   0
Unrealized Losses   0
Fair Value   111,501
U.S. Treasury Securities and Agency Bonds [Member] | Short-term Investments [Member]    
Cash And Cash Equivalents [Line Items]    
Amortized Cost   143,895
Unrealized Gains   0
Unrealized Losses   (210)
Fair Value   143,685
Equity and U.S. Treasury Securities [Member] | Long-term Investments [Member]    
Cash And Cash Equivalents [Line Items]    
Amortized Cost   16,538
Unrealized Gains   0
Unrealized Losses   (20)
Fair Value   $ 16,518