Annual report pursuant to Section 13 and 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
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]      
Cost $ 69,277    
Gross Unrealized Losses (134)    
Fair Value 69,143    
Cash and cash equivalents 29,545 $ 12,078 $ 19,197
Marketable securities 39,598    
Cash [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Cost 8,925    
Fair Value 8,925    
Cash and cash equivalents 8,925 9,922  
Level 1 [Member] | Money Market Funds [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Cost 20,620    
Fair Value 20,620    
Cash and cash equivalents 20,620 $ 2,156  
Level 2 [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Cost 39,732    
Gross Unrealized Losses (134)    
Fair Value 39,598    
Marketable securities 39,598    
U.S. Government Securities [Member] | Level 2 [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Cost 4,505    
Gross Unrealized Losses (17)    
Fair Value 4,488    
Marketable securities 4,488    
Commercial Papers [Member] | Level 2 [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Cost 4,959    
Gross Unrealized Losses (5)    
Fair Value 4,954    
Marketable securities 4,954    
Corporate Debt Securities [Member] | Level 2 [Member]      
Schedule of Available-for-sale Securities [Line Items]      
Cost 30,268    
Gross Unrealized Losses (112)    
Fair Value 30,156    
Marketable securities $ 30,156