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International Activities - Additional Information (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
International Activities [Line Items]      
Revenues from international trust-related services $ 32,000 $ 29,000 $ 24,000
Loans to foreign borrowers 89,871,798 87,447,033  
Deposits at foreign office 1,684,044 811,906  
Cayman Islands [Member]      
International Activities [Line Items]      
Deposits at foreign office 1,680,000 812,000  
Canada [Member]      
International Activities [Line Items]      
Deposits at foreign office 23,000 22,000  
Geographic Distribution, Foreign [Member]      
International Activities [Line Items]      
Loans to foreign borrowers $ 186,000 $ 172,000  
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%