Quarterly report pursuant to Section 13 or 15(d)

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

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Summary of Significant Accounting Policies - 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, 2017
Dec. 31, 2016
Sep. 30, 2016
Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]        
Cost $ 60,494      
Gross Unrealized Losses (62)      
Fair Value 60,432      
Cash and cash equivalents 20,762 $ 12,078 $ 7,626 $ 19,197
Marketable securities 39,670      
Cash [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cost 13,711      
Fair Value 13,711      
Cash and cash equivalents 13,711      
Level 1 [Member] | Money Market Funds [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cost 3,057      
Fair Value 3,057      
Cash and cash equivalents 3,057      
Level 2 [Member] | Corporate Securities [Member]        
Schedule of Available-for-sale Securities [Line Items]        
Cost 43,726      
Gross Unrealized Losses (62)      
Fair Value 43,664      
Cash and cash equivalents 3,994      
Marketable securities $ 39,670