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Other Financial Assets and Financial Liabilities - Additional Information (Detail)
฿ in Millions
 12 Months Ended
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Dec. 31, 2015
USD ($)
Dec. 31, 2016
THB (฿)
Disclosure Of Financial Instruments [Line Items]        
Subordinated debentures of Bangkok Bank Public Company Limited   $ 309,000   ฿ 11
Subordinated debentures of Bangkok Bank Public Company Limited, fixed interest rate   4.375%    
Subordinated debentures of Bangkok Bank Public Company Limited, maturity   10 years    
Dividends received $ 100,000 $ 96,000 $ 99,000  
Maximum borrowing capacity 250,923,000 233,837,000    
Unused portion of the credit lines 144,000,000      
Outstanding forward contracts, recognized gain (loss) $ (332,000) $ 171,000 20,000  
Liquidity discount 30.00% 30.00%    
Line Of Credit 1 [Member]        
Disclosure Of Financial Instruments [Line Items]        
Unused portion of the credit lines $ 144,478,000 $ 139,854,000    
Letters of Credit [Member]        
Disclosure Of Financial Instruments [Line Items]        
Unused portion of the credit lines 74,911,000 74,039,000    
Open letters of credit [Member]        
Disclosure Of Financial Instruments [Line Items]        
Unused portion of the credit lines 37,559,000 24,743,000    
Foreign currency forward contracts [Member]        
Disclosure Of Financial Instruments [Line Items]        
Outstanding forward contracts, notional amount (18,900,000) 9,500,000    
Outstanding forward contracts, recognized gain (loss) $ (332,000) $ 171,000 $ 20,000