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
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]        
Cost $ 54,715 $ 69,277    
Gross Unrealized Losses (197) (134)    
Fair Value 54,518 69,143    
Cash and cash equivalents 21,026 29,545 $ 8,782 $ 12,078
Marketable securities 33,492 39,598    
Cash [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cost 4,590 8,925    
Fair Value 4,590 8,925    
Cash and cash equivalents 4,590 8,925    
Level 1 [Member] | Money Market Funds [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cost 16,436 20,620    
Fair Value 16,436 20,620    
Cash and cash equivalents 16,436 20,620    
U.S. Government Securities [Member] | Level 2 [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cost 4,501 4,505    
Gross Unrealized Losses (19) (17)    
Fair Value 4,482 4,488    
Marketable securities 4,482 4,488    
Commercial Papers [Member] | Level 2 [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cost 2,992 4,959    
Gross Unrealized Losses (5) (5)    
Fair Value 2,987 4,954    
Marketable securities 2,987 4,954    
Corporate Debt Securities [Member] | Level 2 [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cost 26,196 30,268    
Gross Unrealized Losses (173) (112)    
Fair Value 26,023 30,156    
Marketable securities 26,023 30,156    
Corporate Securities [Member] | Level 2 [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cost 33,689 39,732    
Gross Unrealized Losses (197) (134)    
Fair Value 33,492 39,598    
Marketable securities $ 33,492 $ 39,598