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Income taxes - Schedule of Reconciliation of Gross Amounts of Unrecognized Tax Benefits, Excluding Accrued Interest and Penalties (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]    
Unrecognized tax benefits, opening $ 1,134 $ 1,668
Decrease as a result of the Act 0 (534)
Gross increase - tax position in prior period 346 0
Unrecognized tax benefits, ending balance $ 1,480 $ 1,134