Annual report pursuant to Section 13 and 15(d)

Notes Payable to Former Stockholders - Schedule of Interest Expense Pertaining to Notes (Detail)

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Notes Payable to Former Stockholders - Schedule of Interest Expense Pertaining to Notes (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Interest Expense [Line Items]      
Interest expense $ 1,496 $ 1,533 $ 1,726
Notes Payable to Former Stockholders [Member]      
Interest Expense [Line Items]      
Interest expense $ 454 $ 502 $ 548