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
Mar. 31, 2017
Dec. 31, 2016
Mar. 31, 2016
Dec. 31, 2015
Schedule Of Available For Sale Securities [Line Items]        
Cash and cash equivalents $ 18,486 $ 22,061 $ 33,009 $ 69,482
Amortized Cost 124,469 139,416    
Gross Unrealized Gains 1 76    
Gross Unrealized (Losses) (63) (26)    
Estimated Fair Value 124,407 139,466    
Available-for-sale securities [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Amortized Cost 113,837 120,424    
Gross Unrealized Gains 1 76    
Gross Unrealized (Losses) (63) (26)    
Estimated Fair Value 113,775 120,474    
Money market funds [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Cash and cash equivalents 10,632 18,992    
Total cash equivalents 10,632 18,992    
Cash equivalents [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Cash and cash equivalents 10,632 18,992    
Total cash equivalents 10,632 18,992    
Commercial paper securities [Member] | Available-for-sale securities [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Amortized Cost 24,397 23,112    
Gross Unrealized Gains 1 73    
Gross Unrealized (Losses) (4)      
Estimated Fair Value 24,394 23,185    
Corporate debt securities [Member] | Available-for-sale securities [Member]        
Schedule Of Available For Sale Securities [Line Items]        
Amortized Cost 6,001 10,005    
Gross Unrealized (Losses)   (1)    
Estimated Fair Value 6,001 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 83,439 87,307    
Gross Unrealized Gains   3    
Gross Unrealized (Losses) (59) (25)    
Estimated Fair Value $ 83,380 $ 87,285