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Cash Equivalents and Investments - Schedule of Securities Classified as Cash, Cash Equivalents and Short-term Investments (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Cash Cash Equivalents And Short Term Investments [Line Items]        
Cash and cash equivalents, Amortized Cost $ 67,745 $ 75,383 [1] $ 71,213 $ 74,598
Cash and cash equivalents, at Carrying Value 67,747      
Cash and cash equivalents, Gross Unrealized Losses (2)      
Cash and cash equivalents, Fair Value 67,745 75,383    
Short-term investments, Amortized Cost 59,793 58,640    
Short-term investments, Gross Unrealized Losses (89) (47)    
Short-term investments, Fair Value 59,704 58,593    
Cash equivalents and short-term investments, Amortized Cost 127,540 134,023    
Cash equivalents and short-term investments, Gross Unrealized Losses (91) (47)    
Cash equivalents and short-term investments, Fair Value 127,449 133,976    
Cash [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Cash and cash equivalents, Amortized Cost 5,506 5,882    
Cash and cash equivalents, Fair Value 5,506 5,882    
Money Market Funds [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Cash and cash equivalents, Amortized Cost 57,491 68,651    
Cash and cash equivalents, Fair Value 57,491 68,651    
U.S. Treasury Securities [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Cash and cash equivalents, Amortized Cost 1,000      
Cash and cash equivalents, Fair Value 1,000      
Corporate Bonds [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Cash and cash equivalents, Amortized Cost 2,550      
Cash and cash equivalents, Gross Unrealized Losses (2)      
Cash and cash equivalents, Fair Value 2,548      
Commercial Paper (Cash Equivalents) [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Cash and cash equivalents, Amortized Cost 1,200 850    
Cash and cash equivalents, Fair Value 1,200 850    
U.S. Treasury Securities [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Short-term investments, Amortized Cost   3,994    
Short-term investments, Gross Unrealized Losses   (1)    
Short-term investments, Fair Value   3,993    
Corporate Bonds [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Short-term investments, Amortized Cost 27,332 26,853    
Short-term investments, Gross Unrealized Losses (60) (26)    
Short-term investments, Fair Value 27,272 26,827    
Commercial Paper (Investments) [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Short-term investments, Amortized Cost 28,512 19,584    
Short-term investments, Gross Unrealized Losses (19) (14)    
Short-term investments, Fair Value 28,493 19,570    
Asset-Backed Securities [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Short-term investments, Amortized Cost 3,949 8,209    
Short-term investments, Gross Unrealized Losses (10) (6)    
Short-term investments, Fair Value $ 3,939 $ 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.