Quarterly report pursuant to Section 13 or 15(d)

Financial Instruments - Schedule of Cash and Available-For-Sale Securities' Cost, Gross Unrealized Losses and Fair Value by Significant Investment Category (Detail)

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Financial Instruments - Schedule of Cash and Available-For-Sale Securities' Cost, Gross Unrealized Losses and Fair Value by Significant Investment Category (Detail) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Jun. 30, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]        
Cost $ 78,354 $ 69,277    
Gross Unrealized Losses (136) (134)    
Fair Value 78,218 69,143    
Cash and cash equivalents 52,692 29,545 $ 65,912 $ 12,078
Marketable securities 25,526 39,598    
Cash [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cost 11,034 8,925    
Fair Value 11,034 8,925    
Cash and cash equivalents 11,034 8,925    
Level 1 [Member] | Money Market Funds [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cost 41,658 20,620    
Fair Value 41,658 20,620    
Cash and cash equivalents 41,658 20,620    
Level 2 [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cost 25,662 39,732    
Gross Unrealized Losses (136) (134)    
Fair Value 25,526 39,598    
Marketable securities 25,526 39,598    
U.S. Government Securities [Member] | Level 2 [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cost 4,501 4,505    
Gross Unrealized Losses (13) (17)    
Fair Value 4,488 4,488    
Marketable securities 4,488 4,488    
Commercial Papers [Member] | Level 2 [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cost 997 4,959    
Gross Unrealized Losses (2) (5)    
Fair Value 995 4,954    
Marketable securities 995 4,954    
Corporate Debt Securities [Member] | Level 2 [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cost 20,164 30,268    
Gross Unrealized Losses (121) (112)    
Fair Value 20,043 30,156    
Marketable securities $ 20,043 $ 30,156