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Tax matters - Tax reforms (Details) - GBP (£)
12 Months Ended
Sep. 01, 2015
Aug. 31, 2015
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Tax reforms                
Income tax rate (as a percent)           30.00% 30.00% 30.00%
Total tax rate (as a percent)           32.12% 30.48% 23.18%
United States                
Tax reforms                
Corporate tax rate (as a percent)           35.00%    
Argentina                
Tax reforms                
Income tax rate (as a percent)           35.00%    
Spain                
Tax reforms                
Limit for the integration of deferred monetized tax assets (as a percent)             25.00% 70.00%
Limit of the tax rate for the application of deductions (as a percent)             50.00%  
United Kingdom                
Tax reforms                
Corporate tax rate (as a percent)           19.00%    
Surcharge on the standard income tax rate for bank profits (as a percent)           8.00%    
Dividend exemption, generating potential tax credit           £ 2,000    
Brazil                
Tax reforms                
Social contribution tax (as a percent) 20.00% 15.00%            
Income tax rate (as a percent)           25.00%    
Total tax rate (as a percent)           45.00%    
Poland                
Tax reforms                
Tax on certain bank assets (as a percent)           0.0366%    
Chile                
Tax reforms                
First Category tax rate (as a percent)             (24.00%) 25.50%
Forecast | United States | Changes in tax rates                
Tax reforms                
Corporate tax rate (as a percent)         21.00%      
Forecast | Argentina | Changes in tax rates                
Tax reforms                
Income tax rate (as a percent)     25.00% 30.00% 30.00%      
Dividend withholding rate     13.00% 7.00% 7.00%      
Forecast | Chile | Changes in tax rates                
Tax reforms                
First Category tax rate (as a percent)         27.00%      
Maximum | United Kingdom                
Tax reforms                
Corporate tax rate (as a percent)           20.00%    
Minimum | United Kingdom                
Tax reforms                
Corporate tax rate (as a percent)           17.00%