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International Activities - Additional Information (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
International Activities [Line Items]      
Revenues from international trust-related services $ 25,000 $ 26,000 $ 31,000
Loans to foreign borrowers 89,864,419 86,533,507  
Deposits at foreign office 201,927 170,170  
Cayman Islands [Member]      
International Activities [Line Items]      
Deposits at foreign office 201,927 170,170  
Canada [Member]      
International Activities [Line Items]      
Deposits at foreign office 50,000 35,000  
Geographic Distribution, Foreign [Member]      
International Activities [Line Items]      
Loans to foreign borrowers $ 292,000 $ 265,000  
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%