Annual report pursuant to Section 13 and 15(d)

Marketable Securities - Summary of Cash Equivalents and Available-for-Sale Securities (Detail)

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Marketable Securities - Summary of Cash Equivalents and Available-for-Sale Securities (Detail) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Schedule Of Available For Sale Securities [Line Items]        
Cash and cash equivalents $ 22,061 $ 69,482 $ 6,030 $ 10,186
Total cash equivalents 18,992 65,937    
Amortized Cost 139,416 205,440    
Gross Unrealized Gains 76 51    
Gross Unrealized (Losses) (26) (36)    
Estimated Fair Value 139,466 205,455    
Available-for-sale securities [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Amortized Cost 120,424 139,504    
Gross Unrealized Gains 76 50    
Gross Unrealized (Losses) (26) (36)    
Estimated Fair Value 120,474 139,518    
Money market funds [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Cash and cash equivalents 18,992 25,070    
Total cash equivalents 18,992 25,070    
Commercial paper securities [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Cash and cash equivalents   2,000    
Total cash equivalents   2,000    
Commercial paper securities [Member] | Available-for-sale securities [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Amortized Cost 23,112 30,667    
Gross Unrealized Gains 73 50    
Estimated Fair Value 23,185 30,717    
Cash equivalents [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Cash and cash equivalents 18,992 65,936    
Gross Unrealized Gains   1    
Total cash equivalents 18,992 65,937    
U.S. government sponsored entity debt securities [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Cash and cash equivalents   38,866    
Gross Unrealized Gains   1    
Total cash equivalents   38,867    
U.S. government sponsored entity debt securities [Member] | Available-for-sale securities [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Amortized Cost 87,307 91,562    
Gross Unrealized Gains 3      
Gross Unrealized (Losses) (25) (24)    
Estimated Fair Value 87,285 91,538    
Corporate debt securities [Member] | Available-for-sale securities [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Amortized Cost 10,005 17,275    
Gross Unrealized (Losses) (1) (12)    
Estimated Fair Value $ 10,004 $ 17,263