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
Sep. 30, 2018
Dec. 31, 2017
Sep. 30, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]        
Cash and cash equivalents $ 46,127 $ 29,545 $ 20,762 $ 12,078
Marketable securities 18,558 39,598    
Cash [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cash and cash equivalents 16,669 8,925    
Fair Value, Cash 16,669 8,925    
Level 1 [Member] | Money Market Funds [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cash and cash equivalents 29,458 20,620    
Fair Value, Cash 29,458 20,620    
Level 2 [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cash and cash equivalents 46,127 29,545    
Marketable securities 18,558 39,598    
Debt Securities 18,638 39,732    
Gross Unrealized Losses (80) (134)    
Fair Value, Debt Securities 18,558 39,598    
Total Cash and Debt Securities 64,765 69,277    
Total Fair Value, Cash and Debt Securities 64,685 69,143    
Level 2 [Member] | U.S. Government Securities [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Marketable securities 3,494 4,488    
Debt Securities 3,501 4,505    
Gross Unrealized Losses (7) (17)    
Fair Value, Debt Securities 3,494 4,488    
Level 2 [Member] | Corporate Debt Securities [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Marketable securities 15,064 30,156    
Debt Securities 15,137 30,268    
Gross Unrealized Losses (73) (112)    
Fair Value, Debt Securities $ 15,064 30,156    
Level 2 [Member] | Commercial Papers [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Marketable securities   4,954    
Debt Securities   4,959    
Gross Unrealized Losses   (5)    
Fair Value, Debt Securities   $ 4,954