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Income Taxes - Additional Information (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Operating Loss Carryforwards [Line Items]      
Tax expense as result of the Tax Act $ 3,900    
Accumulated other comprehensive income, deferred tax (benefit) expense (1,400) $ (3,000) $ (2,500)
Accumulated other comprehensive income, a deferred tax expense (benefit) (586) 2,300 $ 866
Retained earnings amount for which no provision for income tax has been made 51,800    
Unrecognized tax liability 13,700    
State [Member] | New Jersey [Member]      
Operating Loss Carryforwards [Line Items]      
Unused capital loss carryforwards 240    
Federal [Member]      
Operating Loss Carryforwards [Line Items]      
Operating loss carryforwards, limitations on use   $ 840  
Beacon Trust [Member]      
Operating Loss Carryforwards [Line Items]      
Unused capital loss carryforwards $ 1,300