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International Activities - Additional Information (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
International Activities [Line Items]      
Revenues from international trust-related services $ 29,000 $ 24,000 $ 25,000
Loans to foreign borrowers 87,447,033 86,971,785  
Deposits at foreign office 811,906 177,996  
Cayman Islands [Member]      
International Activities [Line Items]      
Deposits at foreign office 812 178  
Canada [Member]      
International Activities [Line Items]      
Deposits at foreign office 22,000 45,000  
Geographic Distribution, Foreign [Member]      
International Activities [Line Items]      
Loans to foreign borrowers $ 172,000 $ 159,000  
Geographic Concentration Risk [Member] | Assets, Total [Member]      
International Activities [Line Items]      
Concentration risk percentage 1.00%    
Geographic Concentration Risk [Member] | Sales Revenue, Net [Member]      
International Activities [Line Items]      
Concentration risk percentage 1.00%