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Income Taxes - Summary of Reconciliation of Uncertain Tax Positions (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]      
Beginning balance $ 486 $ 425 $ 306
Addition for current year tax positions 101 100 74
Addition for prior year tax positions 16 2 45
Reduction for prior year positions (52) 0 0
Reduction for prior year positions resolved during the current year (2) (41) 0
Ending balance $ 549 $ 486 $ 425