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Cash, Cash Equivalents and Investments - Schedule of Securities Classified as Cash, Cash Equivalents and Short-term Investments (Detail) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Dec. 31, 2016
Cash Cash Equivalents And Short Term Investments [Line Items]        
Cash and cash equivalents, Amortized Cost $ 75,015 $ 75,383 [1] $ 59,454 $ 74,598
Cash and cash equivalents, Fair Value 75,015 75,383    
Short-term investments, Amortized Cost 111,408 58,640    
Short-term investments, Gross Unrealized Losses (17) (47)    
Short-term investments, Fair Value 111,391 58,593    
Cash equivalents and short-term investments, Amortized Cost 186,423 134,023    
Cash equivalents and short-term investments, Gross Unrealized Losses (17) (47)    
Cash equivalents and short-term investments, Fair Value 186,406 133,976    
Cash [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Cash and cash equivalents, Amortized Cost 2,787 5,882    
Cash and cash equivalents, Fair Value 2,787 5,882    
Money Market Funds [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Cash and cash equivalents, Amortized Cost 66,235 68,651    
Cash and cash equivalents, Fair Value 66,235 68,651    
Corporate Bonds (Investments) [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Cash and cash equivalents, Amortized Cost 1,999      
Cash and cash equivalents, Fair Value 1,999      
Commercial Paper (Cash Equivalents) [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Cash and cash equivalents, Amortized Cost 3,994 850    
Cash and cash equivalents, Fair Value 3,994 850    
U.S. Treasury Securities [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Short-term investments, Amortized Cost 7,959 3,994    
Short-term investments, Gross Unrealized Losses (2) (1)    
Short-term investments, Fair Value 7,957 3,993    
Corporate Bonds (Investments) [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Short-term investments, Amortized Cost 34,449 26,853    
Short-term investments, Gross Unrealized Losses (9) (26)    
Short-term investments, Fair Value 34,440 26,827    
Commercial Paper (Investments) [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Short-term investments, Amortized Cost 61,116 19,584    
Short-term investments, Gross Unrealized Losses (5) (14)    
Short-term investments, Fair Value 61,111 19,570    
Asset-Backed Securities [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Short-term investments, Amortized Cost 7,884 8,209    
Short-term investments, Gross Unrealized Losses (1) (6)    
Short-term investments, Fair Value $ 7,883 $ 8,203    
[1] Derived from the audited consolidated financial statements included in the Company’s Annual Report on Form 10K for the year ended December 31, 2017.