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Summary of Significant Accounting Policies - Additional Information (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Jan. 01, 2020
Dec. 31, 2002
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Property, Plant and Equipment [Line Items]            
Cash and cash equivalents maturity period, days     90 days      
Allowance for credit losses     $ 101,466 $ 55,525 $ 55,562 $ 60,195
Credit loss expense for off-balance sheet credit exposures     29,700      
Retained earnings     718,090 695,273    
Individually evaluated for impairment     $ 8,974 $ 5,067    
Percentage of participants matched contribution     6.00%      
Dividends paid on unallocated ESOP shares over period     30 years      
Shares held by the DDFP     180,897      
Dividends paid on unallocated ESOP shares over period   10 years        
Cumulative Effect, Period of Adoption, Adjustment            
Property, Plant and Equipment [Line Items]            
Allowance for credit losses $ 7,920          
Retained earnings 8,300          
Accounting Standards Update 2016-13            
Property, Plant and Equipment [Line Items]            
Allowance for credit losses 7,900          
Credit loss expense for off-balance sheet credit exposures 3,200          
Retained earnings 8,300          
Accounting Standards Update 2016-13 | Construction            
Property, Plant and Equipment [Line Items]            
Allowance for credit losses $ 1,300          
Depositor Relationships            
Property, Plant and Equipment [Line Items]            
Amortized accelerated basis period     8 years 9 months 18 days      
Estimated useful life of core deposits     10 years      
Minimum            
Property, Plant and Equipment [Line Items]            
Individual loans evaluated for impairment, minimum balance     $ 1,000      
Beacon Trust | Customer Relationships            
Property, Plant and Equipment [Line Items]            
Amortized accelerated basis period     12 years      
The MDE Group | Customer Relationships            
Property, Plant and Equipment [Line Items]            
Amortized accelerated basis period     10 years 4 months 24 days      
Tirschwell & Loewy, Inc. | Customer Relationships            
Property, Plant and Equipment [Line Items]            
Amortized accelerated basis period     10 years      
SB One Bancorp            
Property, Plant and Equipment [Line Items]            
Allowance for credit losses     $ 13,600      
SB One Bancorp | Customer Relationships            
Property, Plant and Equipment [Line Items]            
Amortized accelerated basis period     13 years