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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, 2017
Dec. 31, 2016
Mar. 31, 2016
Dec. 31, 2015
Cash Cash Equivalents And Short Term Investments [Line Items]        
Cash and cash equivalents, Amortized Cost $ 71,213 $ 74,598 [1] $ 171,678 $ 107,004
Cash and cash equivalents, Gross Unrealized Gains 0 0    
Cash and cash equivalents, Gross Unrealized Losses 0 0    
Cash and cash equivalents, Fair Value 71,213 74,598    
Short-term investments, Amortized Cost 102,258 126,296    
Short-term investments, Gross Unrealized Gains 5 7    
Short-term investments, Gross Unrealized Losses (53) (78)    
Short-term investments, Fair Value 102,210 126,225    
Cash equivalents and short-term investments, Amortized Cost 173,471 200,894    
Cash equivalents and short-term investments, Gross Unrealized Gains 5 7    
Cash equivalents and short-term investments, Gross Unrealized Losses (53) (78)    
Cash equivalents and short-term investments, Fair Value 173,423 200,823    
Cash [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Cash and cash equivalents, Amortized Cost 1,084 3,638    
Cash and cash equivalents, Gross Unrealized Gains 0 0    
Cash and cash equivalents, Gross Unrealized Losses 0 0    
Cash and cash equivalents, Fair Value 1,084 3,638    
Money Market Funds [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Cash and cash equivalents, Amortized Cost 70,129 68,561    
Cash and cash equivalents, Gross Unrealized Gains 0 0    
Cash and cash equivalents, Gross Unrealized Losses 0 0    
Cash and cash equivalents, Fair Value 70,129 68,561    
Commercial Paper (Cash Equivalents) [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Cash and cash equivalents, Amortized Cost   2,399    
Cash and cash equivalents, Gross Unrealized Gains   0    
Cash and cash equivalents, Gross Unrealized Losses   0    
Cash and cash equivalents, Fair Value   2,399    
Corporate Bonds (Investments) [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Short-term investments, Amortized Cost 46,098 58,464    
Short-term investments, Gross Unrealized Gains 2 2    
Short-term investments, Gross Unrealized Losses (33) (56)    
Short-term investments, Fair Value 46,067 58,410    
Commercial Paper (Investments) [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Short-term investments, Amortized Cost 41,792 62,946    
Short-term investments, Gross Unrealized Gains 2 5    
Short-term investments, Gross Unrealized Losses (7) (20)    
Short-term investments, Fair Value 41,787 62,931    
Asset-Backed Securities [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Short-term investments, Amortized Cost 14,368 4,886    
Short-term investments, Gross Unrealized Gains 1      
Short-term investments, Gross Unrealized Losses (13) (2)    
Short-term investments, Fair Value $ 14,356 $ 4,884    
[1] Derived from the audited financial statements included in the Company's Annual Report on Form 10-K for the year ended December 31, 2016.