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Cash, Cash Equivalents and Investments - Schedule of Securities Classified as Cash and Cash Equivalents and Short-term Investments (Detail) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Cash Cash Equivalents And Short Term Investments [Line Items]        
Cash and cash equivalents, Amortized Cost $ 74,598 $ 107,004 $ 107,286 $ 34,435
Cash and cash equivalents, Gross Unrealized Gains 0 0    
Cash and cash equivalents, Gross Unrealized Losses 0 0    
Cash and cash equivalents, Fair Value 74,598 107,004    
Short-term investments, Amortized Cost 126,296      
Short-term investments, Gross Unrealized Gains 7      
Short-term investments, Gross Unrealized Losses (78)      
Short-term investments, Fair Value 126,225      
Cash equivalents and short-term investments, Amortized Cost 200,894      
Cash equivalents and short-term investments, Gross Unrealized Gains 7      
Cash equivalents and short-term investments, Gross Unrealized Losses (78)      
Cash equivalents and short-term investments, Fair Value 200,823      
Cash [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Cash and cash equivalents, Amortized Cost 3,638 25,194    
Cash and cash equivalents, Gross Unrealized Gains 0 0    
Cash and cash equivalents, Gross Unrealized Losses 0 0    
Cash and cash equivalents, Fair Value 3,638 25,194    
Money Market Funds [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Cash and cash equivalents, Amortized Cost 68,561 81,810    
Cash and cash equivalents, Gross Unrealized Gains 0 0    
Cash and cash equivalents, Gross Unrealized Losses 0 0    
Cash and cash equivalents, Fair Value 68,561 $ 81,810    
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 58,464      
Short-term investments, Gross Unrealized Gains 2      
Short-term investments, Gross Unrealized Losses (56)      
Short-term investments, Fair Value 58,410      
Commercial Paper (Investments) [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Short-term investments, Amortized Cost 62,946      
Short-term investments, Gross Unrealized Gains 5      
Short-term investments, Gross Unrealized Losses (20)      
Short-term investments, Fair Value 62,931      
Asset-Backed Securities [Member]        
Cash Cash Equivalents And Short Term Investments [Line Items]        
Short-term investments, Amortized Cost 4,886      
Short-term investments, Gross Unrealized Losses (2)      
Short-term investments, Fair Value $ 4,884