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Reconciliation of Unrecognized Tax Benefits (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Income Tax Disclosure [Abstract]    
Unrecognized tax benefits at beginning of period $ 10,836 $ 0
Increases related to prior year tax positions 19 10,691
Increases related to current year tax positions 207 145
Unrecognized tax benefits at end of period $ 11,062 $ 10,836