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Accumulated Other Comprehensive Loss - Schedule of Components of Accumulated Other Comprehensive Loss (Detail) (Parenthetical) (Detail) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Other Comprehensive Income Loss Tax Parenthetical Disclosures [Abstract]        
Tax expense (benefit) on unrealized gain (loss) on investments $ 21 $ 14 $ 4 $ 29