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Regulatory capital requirements - Additional Information (Details) - Standardized approach - Basel III
Dec. 31, 2019
Dec. 31, 2018
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]    
CET 1 minimum ratio required ratio 6.50% 6.50%
Tier one risk based capital required for capital adequacy ratio 8.00% 8.00%
Capital required for capital adequacy ratio 10.00% 10.00%
Leverage ratio required 5.00% 5.00%