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Capital Management, Corporate Governance Transparency Policy and Risk Management - Summary of Bank's Internal Credit Rating Grades (Detail)
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Disclosure of internal credit grades [line items]    
% of Gross Carrying Amount 100.00% 100.00%
Performing [member]    
Disclosure of internal credit grades [line items]    
% of Gross Carrying Amount 96.17% 95.45%
Weighted PD 2.32% 2.60%
High grade [member]    
Disclosure of internal credit grades [line items]    
% of Gross Carrying Amount 76.20% 76.40%
Weighted PD 1.13% 1.25%
Standard grade [member]    
Disclosure of internal credit grades [line items]    
% of Gross Carrying Amount 12.64% 8.23%
Weighted PD 4.79% 4.83%
Sub-standard grade [member]    
Disclosure of internal credit grades [line items]    
% of Gross Carrying Amount 7.33% 10.82%
Weighted PD 12.48% 11.64%
Past Due But Not Impaired [member]    
Disclosure of internal credit grades [line items]    
% of Gross Carrying Amount 2.70% 2.84%
Weighted PD 33.24% 32.13%
Impaired [Member]    
Disclosure of internal credit grades [line items]    
% of Gross Carrying Amount 1.13% 1.71%
Weighted PD 100.00% 100.00%