Quarterly report pursuant to Section 13 or 15(d)

Marketable Securities - Schedule of Investments (Detail)

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Marketable Securities - Schedule of Investments (Detail) - USD ($)
$ in Thousands
Jun. 30, 2017
Dec. 31, 2016
Jun. 30, 2016
Dec. 31, 2015
Schedule Of Available For Sale Securities [Line Items]        
Cash and cash equivalents $ 69,837 $ 22,061 $ 12,955 $ 69,482
Amortized Cost 257,915 139,416    
Gross Unrealized Gains   76    
Gross Unrealized (Losses) (113) (26)    
Estimated Fair Value 257,802 139,466    
Available-for-sale securities [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Amortized Cost 196,530 120,424    
Gross Unrealized Gains   76    
Gross Unrealized (Losses) (113) (26)    
Estimated Fair Value 196,417 120,474    
Money market funds [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Cash and cash equivalents 61,385 18,992    
Total cash equivalents 61,385 18,992    
Cash equivalents [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Cash and cash equivalents 61,385 18,992    
Total cash equivalents 61,385 18,992    
Commercial paper securities [Member] | Available-for-sale securities [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Amortized Cost 65,465 23,112    
Gross Unrealized Gains   73    
Gross Unrealized (Losses) (21)      
Estimated Fair Value 65,444 23,185    
Corporate debt securities [Member] | Available-for-sale securities [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Amortized Cost 64,943 10,005    
Gross Unrealized (Losses) (36) (1)    
Estimated Fair Value 64,907 10,004    
U.S. government sponsored entity debt securities [Member] | Available-for-sale securities [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Amortized Cost 66,122 87,307    
Gross Unrealized Gains   3    
Gross Unrealized (Losses) (56) (25)    
Estimated Fair Value $ 66,066 $ 87,285