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Credit quality of loans and advances to banks and customers (Details) - GBP (£)
£ in Millions
Jun. 30, 2020
Dec. 31, 2019
Dec. 31, 2018
Impairment      
Internal credit ratings systems      
Financial assets £ (6,541) £ (3,455)  
Impairment | Purchased or originated credit-impaired      
Internal credit ratings systems      
Financial assets (325) (142)  
Impairment | 12-month expected credit losses      
Internal credit ratings systems      
Financial assets (1,606) (772)  
Impairment | Lifetime expected credit losses | Not credit impaired      
Internal credit ratings systems      
Financial assets (2,384) (1,072)  
Impairment | Lifetime expected credit losses | Credit-impaired after purchase or origination      
Internal credit ratings systems      
Financial assets (2,226) (1,469)  
Loans and advances to banks      
Internal credit ratings systems      
Financial assets £ 11,202 £ 9,775  
Loans and advances to banks | CMS 1-10 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 0.00% 0.00%  
Expected credit loss PD (as a percent) 0.00% 0.00%  
Loans and advances to banks | CMS 1-10 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 0.50% 0.50%  
Expected credit loss PD (as a percent) 0.50% 0.50%  
Loans and advances to banks | CMS 11-14 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 0.51% 0.51%  
Expected credit loss PD (as a percent) 0.51% 0.51%  
Loans and advances to banks | CMS 11-14 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 3.00% 3.00%  
Expected credit loss PD (as a percent) 3.00% 3.00%  
Loans and advances to banks | CMS 15-18 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 3.01% 3.01%  
Expected credit loss PD (as a percent) 3.01% 3.01%  
Loans and advances to banks | CMS 15-18 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 20.00% 20.00%  
Expected credit loss PD (as a percent) 20.00% 20.00%  
Loans and advances to banks | CMS 19 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 20.01% 20.01%  
Expected credit loss PD (as a percent) 20.01% 20.01%  
Loans and advances to banks | CMS 19 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 99.99% 99.99%  
Expected credit loss PD (as a percent) 99.99% 99.99%  
Loans and advances to banks | CMS 20-23      
Internal credit ratings systems      
Regulatory PD (as a percent) 100.00% 100.00%  
Expected credit loss PD (as a percent) 100.00% 100.00%  
Loans and advances to banks | 12-month expected credit losses      
Internal credit ratings systems      
Financial assets £ 11,159 £ 9,775  
Loans and advances to banks | Lifetime expected credit losses | Not credit impaired      
Internal credit ratings systems      
Financial assets 43    
Loans and advances to banks | Gross carrying amount      
Internal credit ratings systems      
Financial assets 11,224 9,777 £ 6,285
Loans and advances to banks | Gross carrying amount | CMS 1-10      
Internal credit ratings systems      
Financial assets 6,094 9,777  
Loans and advances to banks | Gross carrying amount | CMS 11-14      
Internal credit ratings systems      
Financial assets 5,049    
Loans and advances to banks | Gross carrying amount | CMS 15-18      
Internal credit ratings systems      
Financial assets 81    
Loans and advances to banks | Gross carrying amount | CMS      
Internal credit ratings systems      
Financial assets 11,224 9,777  
Loans and advances to banks | Gross carrying amount | 12-month expected credit losses      
Internal credit ratings systems      
Financial assets 11,180 9,777 6,282
Loans and advances to banks | Gross carrying amount | 12-month expected credit losses | CMS 1-10      
Internal credit ratings systems      
Financial assets 6,094 9,777  
Loans and advances to banks | Gross carrying amount | 12-month expected credit losses | CMS 11-14      
Internal credit ratings systems      
Financial assets 5,049    
Loans and advances to banks | Gross carrying amount | 12-month expected credit losses | CMS 15-18      
Internal credit ratings systems      
Financial assets 37    
Loans and advances to banks | Gross carrying amount | 12-month expected credit losses | CMS      
Internal credit ratings systems      
Financial assets 11,180 9,777  
Loans and advances to banks | Gross carrying amount | Lifetime expected credit losses | Not credit impaired      
Internal credit ratings systems      
Financial assets 44   3
Loans and advances to banks | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | CMS 15-18      
Internal credit ratings systems      
Financial assets 44    
Loans and advances to banks | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | CMS      
Internal credit ratings systems      
Financial assets 44    
Loans and advances to banks | Impairment      
Internal credit ratings systems      
Financial assets (22) (2)  
Loans and advances to banks | Impairment | CMS 1-10      
Internal credit ratings systems      
Financial assets (1) (2)  
Loans and advances to banks | Impairment | CMS 11-14      
Internal credit ratings systems      
Financial assets (19)    
Loans and advances to banks | Impairment | CMS 15-18      
Internal credit ratings systems      
Financial assets (2)    
Loans and advances to banks | Impairment | CMS      
Internal credit ratings systems      
Financial assets (22) (2)  
Loans and advances to banks | Impairment | 12-month expected credit losses      
Internal credit ratings systems      
Financial assets (21) (2)  
Loans and advances to banks | Impairment | 12-month expected credit losses | CMS 1-10      
Internal credit ratings systems      
Financial assets (1) (2)  
Loans and advances to banks | Impairment | 12-month expected credit losses | CMS 11-14      
Internal credit ratings systems      
Financial assets (19)    
Loans and advances to banks | Impairment | 12-month expected credit losses | CMS 15-18      
Internal credit ratings systems      
Financial assets (1)    
Loans and advances to banks | Impairment | 12-month expected credit losses | CMS      
Internal credit ratings systems      
Financial assets (21) (2)  
Loans and advances to banks | Impairment | Lifetime expected credit losses | Not credit impaired      
Internal credit ratings systems      
Financial assets (1)    
Loans and advances to banks | Impairment | Lifetime expected credit losses | Not credit impaired | CMS 15-18      
Internal credit ratings systems      
Financial assets (1)    
Loans and advances to banks | Impairment | Lifetime expected credit losses | Not credit impaired | CMS      
Internal credit ratings systems      
Financial assets (1)    
Loans and advances to customers      
Internal credit ratings systems      
Financial assets 501,508 494,988  
Loans and advances to customers | Purchased or originated credit-impaired      
Internal credit ratings systems      
Financial assets 12,718 13,572  
Loans and advances to customers | 12-month expected credit losses      
Internal credit ratings systems      
Financial assets 428,197 449,300  
Loans and advances to customers | Lifetime expected credit losses | Not credit impaired      
Internal credit ratings systems      
Financial assets 55,971 27,548  
Loans and advances to customers | Lifetime expected credit losses | Credit-impaired after purchase or origination      
Internal credit ratings systems      
Financial assets 4,622 4,568  
Loans and advances to customers | Gross carrying amount      
Internal credit ratings systems      
Financial assets 507,494 498,247 488,008
Loans and advances to customers | Gross carrying amount | Purchased or originated credit-impaired      
Internal credit ratings systems      
Financial assets 13,043 13,714 15,391
Loans and advances to customers | Gross carrying amount | 12-month expected credit losses      
Internal credit ratings systems      
Financial assets 429,529 449,975 441,531
Loans and advances to customers | Gross carrying amount | Lifetime expected credit losses | Not credit impaired      
Internal credit ratings systems      
Financial assets 58,139 28,543 25,345
Loans and advances to customers | Gross carrying amount | Lifetime expected credit losses | Credit-impaired after purchase or origination      
Internal credit ratings systems      
Financial assets 6,783 6,015 £ 5,741
Loans and advances to customers | Impairment      
Internal credit ratings systems      
Financial assets (5,986) (3,259)  
Loans and advances to customers | Impairment | Purchased or originated credit-impaired      
Internal credit ratings systems      
Financial assets (325) (142)  
Loans and advances to customers | Impairment | 12-month expected credit losses      
Internal credit ratings systems      
Financial assets (1,332) (675)  
Loans and advances to customers | Impairment | Lifetime expected credit losses | Not credit impaired      
Internal credit ratings systems      
Financial assets (2,168) (995)  
Loans and advances to customers | Impairment | Lifetime expected credit losses | Credit-impaired after purchase or origination      
Internal credit ratings systems      
Financial assets (2,161) (1,447)  
Loans and advances to customers | Retail | Gross carrying amount      
Internal credit ratings systems      
Financial assets 344,232 344,483  
Loans and advances to customers | Retail | Gross carrying amount | Purchased or originated credit-impaired      
Internal credit ratings systems      
Financial assets 13,043 13,714  
Loans and advances to customers | Retail | Gross carrying amount | 12-month expected credit losses      
Internal credit ratings systems      
Financial assets 286,929 305,751  
Loans and advances to customers | Retail | Gross carrying amount | Lifetime expected credit losses | Not credit impaired      
Internal credit ratings systems      
Financial assets 41,376 22,534  
Loans and advances to customers | Retail | Gross carrying amount | Lifetime expected credit losses | Credit-impaired after purchase or origination      
Internal credit ratings systems      
Financial assets 2,884 2,484  
Loans and advances to customers | Retail | Impairment      
Internal credit ratings systems      
Financial assets (3,184) (1,961)  
Loans and advances to customers | Retail | Impairment | Purchased or originated credit-impaired      
Internal credit ratings systems      
Financial assets (325) (142)  
Loans and advances to customers | Retail | Impairment | 12-month expected credit losses      
Internal credit ratings systems      
Financial assets (850) (563)  
Loans and advances to customers | Retail | Impairment | Lifetime expected credit losses | Not credit impaired      
Internal credit ratings systems      
Financial assets (1,376) (766)  
Loans and advances to customers | Retail | Impairment | Lifetime expected credit losses | Credit-impaired after purchase or origination      
Internal credit ratings systems      
Financial assets £ (633) £ (490)  
Loans and advances to customers | Retail - Mortgages [Member] | RMS 1-6 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 0.00% 0.00%  
Expected credit loss PD (as a percent) 0.00% 0.00%  
Loans and advances to customers | Retail - Mortgages [Member] | RMS 1-6 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 4.50% 4.50%  
Expected credit loss PD (as a percent) 4.50% 4.50%  
Loans and advances to customers | Retail - Mortgages [Member] | RMS 7-9 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 4.51% 4.51%  
Expected credit loss PD (as a percent) 4.51% 4.51%  
Loans and advances to customers | Retail - Mortgages [Member] | RMS 7-9 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 14.00% 14.00%  
Expected credit loss PD (as a percent) 14.00% 14.00%  
Loans and advances to customers | Retail - Mortgages [Member] | RMS 10 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 14.01% 14.01%  
Expected credit loss PD (as a percent) 14.01% 14.01%  
Loans and advances to customers | Retail - Mortgages [Member] | RMS 10 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 20.00% 20.00%  
Expected credit loss PD (as a percent) 20.00% 20.00%  
Loans and advances to customers | Retail - Mortgages [Member] | RMS 11-13 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 20.01% 20.01%  
Expected credit loss PD (as a percent) 20.01% 20.01%  
Loans and advances to customers | Retail - Mortgages [Member] | RMS 11-13 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 99.99% 99.99%  
Expected credit loss PD (as a percent) 99.99% 99.99%  
Loans and advances to customers | Retail - Mortgages [Member] | RMS 14      
Internal credit ratings systems      
Regulatory PD (as a percent) 100.00% 100.00%  
Expected credit loss PD (as a percent) 100.00% 100.00%  
Loans and advances to customers | Retail - Mortgages [Member] | Gross carrying amount | RMS 1-6      
Internal credit ratings systems      
Financial assets £ 263,890 £ 270,522  
Loans and advances to customers | Retail - Mortgages [Member] | Gross carrying amount | RMS 7-9      
Internal credit ratings systems      
Financial assets 3,778 2,067  
Loans and advances to customers | Retail - Mortgages [Member] | Gross carrying amount | RMS 10      
Internal credit ratings systems      
Financial assets 862 414  
Loans and advances to customers | Retail - Mortgages [Member] | Gross carrying amount | RMS 11-13      
Internal credit ratings systems      
Financial assets 2,354 975  
Loans and advances to customers | Retail - Mortgages [Member] | Gross carrying amount | RMS 14      
Internal credit ratings systems      
Financial assets 14,843 15,220  
Loans and advances to customers | Retail - Mortgages [Member] | Gross carrying amount | RMS      
Internal credit ratings systems      
Financial assets 285,727 289,198  
Loans and advances to customers | Retail - Mortgages [Member] | Gross carrying amount | Purchased or originated credit-impaired | RMS 14      
Internal credit ratings systems      
Financial assets 13,043 13,714  
Loans and advances to customers | Retail - Mortgages [Member] | Gross carrying amount | Purchased or originated credit-impaired | RMS      
Internal credit ratings systems      
Financial assets 13,043 13,714  
Loans and advances to customers | Retail - Mortgages [Member] | Gross carrying amount | 12-month expected credit losses | RMS 1-6      
Internal credit ratings systems      
Financial assets 236,569 257,028  
Loans and advances to customers | Retail - Mortgages [Member] | Gross carrying amount | 12-month expected credit losses | RMS 7-9      
Internal credit ratings systems      
Financial assets 8 15  
Loans and advances to customers | Retail - Mortgages [Member] | Gross carrying amount | 12-month expected credit losses | RMS      
Internal credit ratings systems      
Financial assets 236,577 257,043  
Loans and advances to customers | Retail - Mortgages [Member] | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | RMS 1-6      
Internal credit ratings systems      
Financial assets 27,321 13,494  
Loans and advances to customers | Retail - Mortgages [Member] | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | RMS 7-9      
Internal credit ratings systems      
Financial assets 3,770 2,052  
Loans and advances to customers | Retail - Mortgages [Member] | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | RMS 10      
Internal credit ratings systems      
Financial assets 862 414  
Loans and advances to customers | Retail - Mortgages [Member] | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | RMS 11-13      
Internal credit ratings systems      
Financial assets 2,354 975  
Loans and advances to customers | Retail - Mortgages [Member] | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | RMS      
Internal credit ratings systems      
Financial assets 34,307 16,935  
Loans and advances to customers | Retail - Mortgages [Member] | Gross carrying amount | Lifetime expected credit losses | Credit-impaired after purchase or origination | RMS 14      
Internal credit ratings systems      
Financial assets 1,800 1,506  
Loans and advances to customers | Retail - Mortgages [Member] | Gross carrying amount | Lifetime expected credit losses | Credit-impaired after purchase or origination | RMS      
Internal credit ratings systems      
Financial assets 1,800 1,506  
Loans and advances to customers | Retail - Mortgages [Member] | Impairment | RMS 1-6      
Internal credit ratings systems      
Financial assets (356) (206)  
Loans and advances to customers | Retail - Mortgages [Member] | Impairment | RMS 7-9      
Internal credit ratings systems      
Financial assets (79) (39)  
Loans and advances to customers | Retail - Mortgages [Member] | Impairment | RMS 10      
Internal credit ratings systems      
Financial assets (28) (13)  
Loans and advances to customers | Retail - Mortgages [Member] | Impairment | RMS 11-13      
Internal credit ratings systems      
Financial assets (134) (46)  
Loans and advances to customers | Retail - Mortgages [Member] | Impairment | RMS 14      
Internal credit ratings systems      
Financial assets (512) (264)  
Loans and advances to customers | Retail - Mortgages [Member] | Impairment | RMS      
Internal credit ratings systems      
Financial assets (1,109) (568)  
Loans and advances to customers | Retail - Mortgages [Member] | Impairment | Purchased or originated credit-impaired | RMS 14      
Internal credit ratings systems      
Financial assets (325) (142)  
Loans and advances to customers | Retail - Mortgages [Member] | Impairment | Purchased or originated credit-impaired | RMS      
Internal credit ratings systems      
Financial assets (325) (142)  
Loans and advances to customers | Retail - Mortgages [Member] | Impairment | 12-month expected credit losses | RMS 1-6      
Internal credit ratings systems      
Financial assets (106) (23)  
Loans and advances to customers | Retail - Mortgages [Member] | Impairment | 12-month expected credit losses | RMS      
Internal credit ratings systems      
Financial assets (106) (23)  
Loans and advances to customers | Retail - Mortgages [Member] | Impairment | Lifetime expected credit losses | Not credit impaired | RMS 1-6      
Internal credit ratings systems      
Financial assets (250) (183)  
Loans and advances to customers | Retail - Mortgages [Member] | Impairment | Lifetime expected credit losses | Not credit impaired | RMS 7-9      
Internal credit ratings systems      
Financial assets (79) (39)  
Loans and advances to customers | Retail - Mortgages [Member] | Impairment | Lifetime expected credit losses | Not credit impaired | RMS 10      
Internal credit ratings systems      
Financial assets (28) (13)  
Loans and advances to customers | Retail - Mortgages [Member] | Impairment | Lifetime expected credit losses | Not credit impaired | RMS 11-13      
Internal credit ratings systems      
Financial assets (134) (46)  
Loans and advances to customers | Retail - Mortgages [Member] | Impairment | Lifetime expected credit losses | Not credit impaired | RMS      
Internal credit ratings systems      
Financial assets (491) (281)  
Loans and advances to customers | Retail - Mortgages [Member] | Impairment | Lifetime expected credit losses | Credit-impaired after purchase or origination | RMS 14      
Internal credit ratings systems      
Financial assets (187) (122)  
Loans and advances to customers | Retail - Mortgages [Member] | Impairment | Lifetime expected credit losses | Credit-impaired after purchase or origination | RMS      
Internal credit ratings systems      
Financial assets £ (187) £ (122)  
Loans and advances to customers | Retail - credit cards | RMS 1-6 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 0.00% 0.00%  
Expected credit loss PD (as a percent) 0.00% 0.00%  
Loans and advances to customers | Retail - credit cards | RMS 1-6 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 4.50% 4.50%  
Expected credit loss PD (as a percent) 4.50% 4.50%  
Loans and advances to customers | Retail - credit cards | RMS 7-9 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 4.51% 4.51%  
Expected credit loss PD (as a percent) 4.51% 4.51%  
Loans and advances to customers | Retail - credit cards | RMS 7-9 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 14.00% 14.00%  
Expected credit loss PD (as a percent) 14.00% 14.00%  
Loans and advances to customers | Retail - credit cards | RMS 10 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 14.01% 14.01%  
Expected credit loss PD (as a percent) 14.01% 14.01%  
Loans and advances to customers | Retail - credit cards | RMS 10 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 20.00% 20.00%  
Expected credit loss PD (as a percent) 20.00% 20.00%  
Loans and advances to customers | Retail - credit cards | RMS 11-13 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 20.01% 20.01%  
Expected credit loss PD (as a percent) 20.01% 20.01%  
Loans and advances to customers | Retail - credit cards | RMS 11-13 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 99.99% 99.99%  
Expected credit loss PD (as a percent) 99.99% 99.99%  
Loans and advances to customers | Retail - credit cards | RMS 14      
Internal credit ratings systems      
Regulatory PD (as a percent) 100.00% 100.00%  
Expected credit loss PD (as a percent) 100.00% 100.00%  
Loans and advances to customers | Retail - credit cards | Gross carrying amount | RMS 1-6      
Internal credit ratings systems      
Financial assets £ 10,526 £ 15,474  
Loans and advances to customers | Retail - credit cards | Gross carrying amount | RMS 7-9      
Internal credit ratings systems      
Financial assets 3,523 1,911  
Loans and advances to customers | Retail - credit cards | Gross carrying amount | RMS 10      
Internal credit ratings systems      
Financial assets 764 137  
Loans and advances to customers | Retail - credit cards | Gross carrying amount | RMS 11-13      
Internal credit ratings systems      
Financial assets 714 292  
Loans and advances to customers | Retail - credit cards | Gross carrying amount | RMS 14      
Internal credit ratings systems      
Financial assets 368 385  
Loans and advances to customers | Retail - credit cards | Gross carrying amount | RMS      
Internal credit ratings systems      
Financial assets 15,895 18,199  
Loans and advances to customers | Retail - credit cards | Gross carrying amount | 12-month expected credit losses | RMS 1-6      
Internal credit ratings systems      
Financial assets 10,070 14,745  
Loans and advances to customers | Retail - credit cards | Gross carrying amount | 12-month expected credit losses | RMS 7-9      
Internal credit ratings systems      
Financial assets 2,882 1,355  
Loans and advances to customers | Retail - credit cards | Gross carrying amount | 12-month expected credit losses | RMS 10      
Internal credit ratings systems      
Financial assets 403 32  
Loans and advances to customers | Retail - credit cards | Gross carrying amount | 12-month expected credit losses | RMS 11-13      
Internal credit ratings systems      
Financial assets 84 1  
Loans and advances to customers | Retail - credit cards | Gross carrying amount | 12-month expected credit losses | RMS      
Internal credit ratings systems      
Financial assets 13,439 16,133  
Loans and advances to customers | Retail - credit cards | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | RMS 1-6      
Internal credit ratings systems      
Financial assets 456 729  
Loans and advances to customers | Retail - credit cards | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | RMS 7-9      
Internal credit ratings systems      
Financial assets 641 556  
Loans and advances to customers | Retail - credit cards | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | RMS 10      
Internal credit ratings systems      
Financial assets 361 105  
Loans and advances to customers | Retail - credit cards | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | RMS 11-13      
Internal credit ratings systems      
Financial assets 630 291  
Loans and advances to customers | Retail - credit cards | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | RMS      
Internal credit ratings systems      
Financial assets 2,088 1,681  
Loans and advances to customers | Retail - credit cards | Gross carrying amount | Lifetime expected credit losses | Credit-impaired after purchase or origination | RMS 14      
Internal credit ratings systems      
Financial assets 368 385  
Loans and advances to customers | Retail - credit cards | Gross carrying amount | Lifetime expected credit losses | Credit-impaired after purchase or origination | RMS      
Internal credit ratings systems      
Financial assets 368 385  
Loans and advances to customers | Retail - credit cards | Impairment | RMS 1-6      
Internal credit ratings systems      
Financial assets (118) (128)  
Loans and advances to customers | Retail - credit cards | Impairment | RMS 7-9      
Internal credit ratings systems      
Financial assets (195) (103)  
Loans and advances to customers | Retail - credit cards | Impairment | RMS 10      
Internal credit ratings systems      
Financial assets (102) (22)  
Loans and advances to customers | Retail - credit cards | Impairment | RMS 11-13      
Internal credit ratings systems      
Financial assets (221) (91)  
Loans and advances to customers | Retail - credit cards | Impairment | RMS 14      
Internal credit ratings systems      
Financial assets (121) (126)  
Loans and advances to customers | Retail - credit cards | Impairment | RMS      
Internal credit ratings systems      
Financial assets (757) (470)  
Loans and advances to customers | Retail - credit cards | Impairment | 12-month expected credit losses | RMS 1-6      
Internal credit ratings systems      
Financial assets (96) (103)  
Loans and advances to customers | Retail - credit cards | Impairment | 12-month expected credit losses | RMS 7-9      
Internal credit ratings systems      
Financial assets (134) (49)  
Loans and advances to customers | Retail - credit cards | Impairment | 12-month expected credit losses | RMS 10      
Internal credit ratings systems      
Financial assets (44) (3)  
Loans and advances to customers | Retail - credit cards | Impairment | 12-month expected credit losses | RMS 11-13      
Internal credit ratings systems      
Financial assets (13)    
Loans and advances to customers | Retail - credit cards | Impairment | 12-month expected credit losses | RMS      
Internal credit ratings systems      
Financial assets (287) (155)  
Loans and advances to customers | Retail - credit cards | Impairment | Lifetime expected credit losses | Not credit impaired | RMS 1-6      
Internal credit ratings systems      
Financial assets (22) (25)  
Loans and advances to customers | Retail - credit cards | Impairment | Lifetime expected credit losses | Not credit impaired | RMS 7-9      
Internal credit ratings systems      
Financial assets (61) (54)  
Loans and advances to customers | Retail - credit cards | Impairment | Lifetime expected credit losses | Not credit impaired | RMS 10      
Internal credit ratings systems      
Financial assets (58) (19)  
Loans and advances to customers | Retail - credit cards | Impairment | Lifetime expected credit losses | Not credit impaired | RMS 11-13      
Internal credit ratings systems      
Financial assets (208) (91)  
Loans and advances to customers | Retail - credit cards | Impairment | Lifetime expected credit losses | Not credit impaired | RMS      
Internal credit ratings systems      
Financial assets (349) (189)  
Loans and advances to customers | Retail - credit cards | Impairment | Lifetime expected credit losses | Credit-impaired after purchase or origination | RMS 14      
Internal credit ratings systems      
Financial assets (121) (126)  
Loans and advances to customers | Retail - credit cards | Impairment | Lifetime expected credit losses | Credit-impaired after purchase or origination | RMS      
Internal credit ratings systems      
Financial assets £ (121) £ (126)  
Loans and advances to customers | Retail - UK Motor Finance [Member] | RMS 1-6 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 0.00% 0.00%  
Expected credit loss PD (as a percent) 0.00% 0.00%  
Loans and advances to customers | Retail - UK Motor Finance [Member] | RMS 1-6 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 4.50% 4.50%  
Expected credit loss PD (as a percent) 4.50% 4.50%  
Loans and advances to customers | Retail - UK Motor Finance [Member] | RMS 7-9 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 4.51% 4.51%  
Expected credit loss PD (as a percent) 4.51% 4.51%  
Loans and advances to customers | Retail - UK Motor Finance [Member] | RMS 7-9 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 14.00% 14.00%  
Expected credit loss PD (as a percent) 14.00% 14.00%  
Loans and advances to customers | Retail - UK Motor Finance [Member] | RMS 10 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 14.01% 14.01%  
Expected credit loss PD (as a percent) 14.01% 14.01%  
Loans and advances to customers | Retail - UK Motor Finance [Member] | RMS 10 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 20.00% 20.00%  
Expected credit loss PD (as a percent) 20.00% 20.00%  
Loans and advances to customers | Retail - UK Motor Finance [Member] | RMS 11-13 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 20.01% 20.01%  
Expected credit loss PD (as a percent) 20.01% 20.01%  
Loans and advances to customers | Retail - UK Motor Finance [Member] | RMS 11-13 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 99.99% 99.99%  
Expected credit loss PD (as a percent) 99.99% 99.99%  
Loans and advances to customers | Retail - UK Motor Finance [Member] | RMS 14      
Internal credit ratings systems      
Regulatory PD (as a percent) 100.00% 100.00%  
Expected credit loss PD (as a percent) 100.00% 100.00%  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Gross carrying amount | RMS 1-6      
Internal credit ratings systems      
Financial assets £ 13,377 £ 14,865  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Gross carrying amount | RMS 7-9      
Internal credit ratings systems      
Financial assets 1,747 682  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Gross carrying amount | RMS 10      
Internal credit ratings systems      
Financial assets 155 99  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Gross carrying amount | RMS 11-13      
Internal credit ratings systems      
Financial assets 315 180  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Gross carrying amount | RMS 14      
Internal credit ratings systems      
Financial assets 236 150  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Gross carrying amount | RMS      
Internal credit ratings systems      
Financial assets 15,830 15,976  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Gross carrying amount | 12-month expected credit losses | RMS 1-6      
Internal credit ratings systems      
Financial assets 11,615 13,568  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Gross carrying amount | 12-month expected credit losses | RMS 7-9      
Internal credit ratings systems      
Financial assets 1,054 314  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Gross carrying amount | 12-month expected credit losses | RMS 11-13      
Internal credit ratings systems      
Financial assets 5 2  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Gross carrying amount | 12-month expected credit losses | RMS      
Internal credit ratings systems      
Financial assets 12,674 13,884  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | RMS 1-6      
Internal credit ratings systems      
Financial assets 1,762 1,297  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | RMS 7-9      
Internal credit ratings systems      
Financial assets 693 368  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | RMS 10      
Internal credit ratings systems      
Financial assets 155 99  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | RMS 11-13      
Internal credit ratings systems      
Financial assets 310 178  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | RMS      
Internal credit ratings systems      
Financial assets 2,920 1,942  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Gross carrying amount | Lifetime expected credit losses | Credit-impaired after purchase or origination | RMS 14      
Internal credit ratings systems      
Financial assets 236 150  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Gross carrying amount | Lifetime expected credit losses | Credit-impaired after purchase or origination | RMS      
Internal credit ratings systems      
Financial assets 236 150  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Impairment | RMS 1-6      
Internal credit ratings systems      
Financial assets (234) (233)  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Impairment | RMS 7-9      
Internal credit ratings systems      
Financial assets (55) (25)  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Impairment | RMS 10      
Internal credit ratings systems      
Financial assets (21) (10)  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Impairment | RMS 11-13      
Internal credit ratings systems      
Financial assets (99) (33)  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Impairment | RMS 14      
Internal credit ratings systems      
Financial assets (152) (84)  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Impairment | RMS      
Internal credit ratings systems      
Financial assets (561) (385)  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Impairment | 12-month expected credit losses | RMS 1-6      
Internal credit ratings systems      
Financial assets (184) (203)  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Impairment | 12-month expected credit losses | RMS 7-9      
Internal credit ratings systems      
Financial assets (8) (10)  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Impairment | 12-month expected credit losses | RMS 11-13      
Internal credit ratings systems      
Financial assets   (1)  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Impairment | 12-month expected credit losses | RMS      
Internal credit ratings systems      
Financial assets (192) (214)  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Impairment | Lifetime expected credit losses | Not credit impaired | RMS 1-6      
Internal credit ratings systems      
Financial assets (50) (30)  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Impairment | Lifetime expected credit losses | Not credit impaired | RMS 7-9      
Internal credit ratings systems      
Financial assets (47) (15)  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Impairment | Lifetime expected credit losses | Not credit impaired | RMS 10      
Internal credit ratings systems      
Financial assets (21) (10)  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Impairment | Lifetime expected credit losses | Not credit impaired | RMS 11-13      
Internal credit ratings systems      
Financial assets (99) (32)  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Impairment | Lifetime expected credit losses | Not credit impaired | RMS      
Internal credit ratings systems      
Financial assets (217) (87)  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Impairment | Lifetime expected credit losses | Credit-impaired after purchase or origination | RMS 14      
Internal credit ratings systems      
Financial assets (152) (84)  
Loans and advances to customers | Retail - UK Motor Finance [Member] | Impairment | Lifetime expected credit losses | Credit-impaired after purchase or origination | RMS      
Internal credit ratings systems      
Financial assets £ (152) £ (84)  
Loans and advances to customers | Retail - Other [Member] | RMS 1-6 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 0.00% 0.00%  
Expected credit loss PD (as a percent) 0.00% 0.00%  
Loans and advances to customers | Retail - Other [Member] | RMS 1-6 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 4.50% 4.50%  
Expected credit loss PD (as a percent) 4.50% 4.50%  
Loans and advances to customers | Retail - Other [Member] | RMS 7-9 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 4.51% 4.51%  
Expected credit loss PD (as a percent) 4.51% 4.51%  
Loans and advances to customers | Retail - Other [Member] | RMS 7-9 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 14.00% 14.00%  
Expected credit loss PD (as a percent) 14.00% 14.00%  
Loans and advances to customers | Retail - Other [Member] | RMS 10 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 14.01% 14.01%  
Expected credit loss PD (as a percent) 14.01% 14.01%  
Loans and advances to customers | Retail - Other [Member] | RMS 10 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 20.00% 20.00%  
Expected credit loss PD (as a percent) 20.00% 20.00%  
Loans and advances to customers | Retail - Other [Member] | RMS 11-13 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 20.01% 20.01%  
Expected credit loss PD (as a percent) 20.01% 20.01%  
Loans and advances to customers | Retail - Other [Member] | RMS 11-13 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 99.99% 99.99%  
Expected credit loss PD (as a percent) 99.99% 99.99%  
Loans and advances to customers | Retail - Other [Member] | RMS 14      
Internal credit ratings systems      
Regulatory PD (as a percent) 100.00% 100.00%  
Expected credit loss PD (as a percent) 100.00% 100.00%  
Loans and advances to customers | Retail - Other [Member] | Gross carrying amount | RMS 1-6      
Internal credit ratings systems      
Financial assets £ 19,935 £ 17,929  
Loans and advances to customers | Retail - Other [Member] | Gross carrying amount | RMS 7-9      
Internal credit ratings systems      
Financial assets 3,759 2,114  
Loans and advances to customers | Retail - Other [Member] | Gross carrying amount | RMS 10      
Internal credit ratings systems      
Financial assets 978 135  
Loans and advances to customers | Retail - Other [Member] | Gross carrying amount | RMS 11-13      
Internal credit ratings systems      
Financial assets 1,628 489  
Loans and advances to customers | Retail - Other [Member] | Gross carrying amount | RMS 14      
Internal credit ratings systems      
Financial assets 480 443  
Loans and advances to customers | Retail - Other [Member] | Gross carrying amount | RMS      
Internal credit ratings systems      
Financial assets 26,780 21,110  
Loans and advances to customers | Retail - Other [Member] | Gross carrying amount | 12-month expected credit losses | RMS 1-6      
Internal credit ratings systems      
Financial assets 19,242 17,166  
Loans and advances to customers | Retail - Other [Member] | Gross carrying amount | 12-month expected credit losses | RMS 7-9      
Internal credit ratings systems      
Financial assets 3,213 1,330  
Loans and advances to customers | Retail - Other [Member] | Gross carrying amount | 12-month expected credit losses | RMS 10      
Internal credit ratings systems      
Financial assets 787 44  
Loans and advances to customers | Retail - Other [Member] | Gross carrying amount | 12-month expected credit losses | RMS 11-13      
Internal credit ratings systems      
Financial assets 997 151  
Loans and advances to customers | Retail - Other [Member] | Gross carrying amount | 12-month expected credit losses | RMS      
Internal credit ratings systems      
Financial assets 24,239 18,691  
Loans and advances to customers | Retail - Other [Member] | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | RMS 1-6      
Internal credit ratings systems      
Financial assets 693 763  
Loans and advances to customers | Retail - Other [Member] | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | RMS 7-9      
Internal credit ratings systems      
Financial assets 546 784  
Loans and advances to customers | Retail - Other [Member] | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | RMS 10      
Internal credit ratings systems      
Financial assets 191 91  
Loans and advances to customers | Retail - Other [Member] | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | RMS 11-13      
Internal credit ratings systems      
Financial assets 631 338  
Loans and advances to customers | Retail - Other [Member] | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | RMS      
Internal credit ratings systems      
Financial assets 2,061 1,976  
Loans and advances to customers | Retail - Other [Member] | Gross carrying amount | Lifetime expected credit losses | Credit-impaired after purchase or origination | RMS 14      
Internal credit ratings systems      
Financial assets 480 443  
Loans and advances to customers | Retail - Other [Member] | Gross carrying amount | Lifetime expected credit losses | Credit-impaired after purchase or origination | RMS      
Internal credit ratings systems      
Financial assets 480 443  
Loans and advances to customers | Retail - Other [Member] | Impairment | RMS 1-6      
Internal credit ratings systems      
Financial assets (144) (135)  
Loans and advances to customers | Retail - Other [Member] | Impairment | RMS 7-9      
Internal credit ratings systems      
Financial assets (153) (119)  
Loans and advances to customers | Retail - Other [Member] | Impairment | RMS 10      
Internal credit ratings systems      
Financial assets (57) (20)  
Loans and advances to customers | Retail - Other [Member] | Impairment | RMS 11-13      
Internal credit ratings systems      
Financial assets (230) (106)  
Loans and advances to customers | Retail - Other [Member] | Impairment | RMS 14      
Internal credit ratings systems      
Financial assets (173) (158)  
Loans and advances to customers | Retail - Other [Member] | Impairment | RMS      
Internal credit ratings systems      
Financial assets (757) (538)  
Loans and advances to customers | Retail - Other [Member] | Impairment | 12-month expected credit losses | RMS 1-6      
Internal credit ratings systems      
Financial assets (116) (109)  
Loans and advances to customers | Retail - Other [Member] | Impairment | 12-month expected credit losses | RMS 7-9      
Internal credit ratings systems      
Financial assets (110) (55)  
Loans and advances to customers | Retail - Other [Member] | Impairment | 12-month expected credit losses | RMS 10      
Internal credit ratings systems      
Financial assets (22) (4)  
Loans and advances to customers | Retail - Other [Member] | Impairment | 12-month expected credit losses | RMS 11-13      
Internal credit ratings systems      
Financial assets (17) (3)  
Loans and advances to customers | Retail - Other [Member] | Impairment | 12-month expected credit losses | RMS      
Internal credit ratings systems      
Financial assets (265) (171)  
Loans and advances to customers | Retail - Other [Member] | Impairment | Lifetime expected credit losses | Not credit impaired | RMS 1-6      
Internal credit ratings systems      
Financial assets (28) (26)  
Loans and advances to customers | Retail - Other [Member] | Impairment | Lifetime expected credit losses | Not credit impaired | RMS 7-9      
Internal credit ratings systems      
Financial assets (43) (64)  
Loans and advances to customers | Retail - Other [Member] | Impairment | Lifetime expected credit losses | Not credit impaired | RMS 10      
Internal credit ratings systems      
Financial assets (35) (16)  
Loans and advances to customers | Retail - Other [Member] | Impairment | Lifetime expected credit losses | Not credit impaired | RMS 11-13      
Internal credit ratings systems      
Financial assets (213) (103)  
Loans and advances to customers | Retail - Other [Member] | Impairment | Lifetime expected credit losses | Not credit impaired | RMS      
Internal credit ratings systems      
Financial assets (319) (209)  
Loans and advances to customers | Retail - Other [Member] | Impairment | Lifetime expected credit losses | Credit-impaired after purchase or origination | RMS 14      
Internal credit ratings systems      
Financial assets (173) (158)  
Loans and advances to customers | Retail - Other [Member] | Impairment | Lifetime expected credit losses | Credit-impaired after purchase or origination | RMS      
Internal credit ratings systems      
Financial assets £ (173) £ (158)  
Loans and advances to customers | Commercial Banking | CMS 1-10 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 0.00% 0.00%  
Expected credit loss PD (as a percent) 0.00% 0.00%  
Loans and advances to customers | Commercial Banking | CMS 1-10 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 0.50% 0.50%  
Expected credit loss PD (as a percent) 0.50% 0.50%  
Loans and advances to customers | Commercial Banking | CMS 11-14 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 0.51% 0.51%  
Expected credit loss PD (as a percent) 0.51% 0.51%  
Loans and advances to customers | Commercial Banking | CMS 11-14 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 3.00% 3.00%  
Expected credit loss PD (as a percent) 3.00% 3.00%  
Loans and advances to customers | Commercial Banking | CMS 15-18 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 3.01% 3.01%  
Expected credit loss PD (as a percent) 3.01% 3.01%  
Loans and advances to customers | Commercial Banking | CMS 15-18 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 20.00% 20.00%  
Expected credit loss PD (as a percent) 20.00% 20.00%  
Loans and advances to customers | Commercial Banking | CMS 19 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 20.01% 20.01%  
Expected credit loss PD (as a percent) 20.01% 20.01%  
Loans and advances to customers | Commercial Banking | CMS 19 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 99.99% 99.99%  
Expected credit loss PD (as a percent) 99.99% 99.99%  
Loans and advances to customers | Commercial Banking | CMS 20-23      
Internal credit ratings systems      
Regulatory PD (as a percent) 100.00% 100.00%  
Expected credit loss PD (as a percent) 100.00% 100.00%  
Loans and advances to customers | Commercial Banking | Gross carrying amount      
Internal credit ratings systems      
Financial assets £ 98,610 £ 96,498  
Loans and advances to customers | Commercial Banking | Gross carrying amount | CMS 1-10      
Internal credit ratings systems      
Financial assets 30,276 60,087  
Loans and advances to customers | Commercial Banking | Gross carrying amount | CMS 11-14      
Internal credit ratings systems      
Financial assets 47,377 27,887  
Loans and advances to customers | Commercial Banking | Gross carrying amount | CMS 15-18      
Internal credit ratings systems      
Financial assets 15,581 4,908  
Loans and advances to customers | Commercial Banking | Gross carrying amount | CMS 19      
Internal credit ratings systems      
Financial assets 1,568 169  
Loans and advances to customers | Commercial Banking | Gross carrying amount | CMS 20-23      
Internal credit ratings systems      
Financial assets 3,808 3,447  
Loans and advances to customers | Commercial Banking | Gross carrying amount | CMS      
Internal credit ratings systems      
Financial assets 98,610 96,498  
Loans and advances to customers | Commercial Banking | Gross carrying amount | 12-month expected credit losses      
Internal credit ratings systems      
Financial assets 78,062 87,074  
Loans and advances to customers | Commercial Banking | Gross carrying amount | 12-month expected credit losses | CMS 1-10      
Internal credit ratings systems      
Financial assets 29,923 59,708  
Loans and advances to customers | Commercial Banking | Gross carrying amount | 12-month expected credit losses | CMS 11-14      
Internal credit ratings systems      
Financial assets 40,101 25,569  
Loans and advances to customers | Commercial Banking | Gross carrying amount | 12-month expected credit losses | CMS 15-18      
Internal credit ratings systems      
Financial assets 8,038 1,797  
Loans and advances to customers | Commercial Banking | Gross carrying amount | 12-month expected credit losses | CMS      
Internal credit ratings systems      
Financial assets 78,062 87,074  
Loans and advances to customers | Commercial Banking | Gross carrying amount | Lifetime expected credit losses | Not credit impaired      
Internal credit ratings systems      
Financial assets 16,740 5,977  
Loans and advances to customers | Commercial Banking | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | CMS 1-10      
Internal credit ratings systems      
Financial assets 353 379  
Loans and advances to customers | Commercial Banking | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | CMS 11-14      
Internal credit ratings systems      
Financial assets 7,276 2,318  
Loans and advances to customers | Commercial Banking | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | CMS 15-18      
Internal credit ratings systems      
Financial assets 7,543 3,111  
Loans and advances to customers | Commercial Banking | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | CMS 19      
Internal credit ratings systems      
Financial assets 1,568 169  
Loans and advances to customers | Commercial Banking | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | CMS      
Internal credit ratings systems      
Financial assets 16,740 5,977  
Loans and advances to customers | Commercial Banking | Gross carrying amount | Lifetime expected credit losses | Credit-impaired after purchase or origination      
Internal credit ratings systems      
Financial assets 3,808 3,447  
Loans and advances to customers | Commercial Banking | Gross carrying amount | Lifetime expected credit losses | Credit-impaired after purchase or origination | CMS 20-23      
Internal credit ratings systems      
Financial assets 3,808 3,447  
Loans and advances to customers | Commercial Banking | Gross carrying amount | Lifetime expected credit losses | Credit-impaired after purchase or origination | CMS      
Internal credit ratings systems      
Financial assets 3,808 3,447  
Loans and advances to customers | Commercial Banking | Impairment      
Internal credit ratings systems      
Financial assets (2,571) (1,265)  
Loans and advances to customers | Commercial Banking | Impairment | CMS 1-10      
Internal credit ratings systems      
Financial assets (23) (34)  
Loans and advances to customers | Commercial Banking | Impairment | CMS 11-14      
Internal credit ratings systems      
Financial assets (295) (87)  
Loans and advances to customers | Commercial Banking | Impairment | CMS 15-18      
Internal credit ratings systems      
Financial assets (508) (187)  
Loans and advances to customers | Commercial Banking | Impairment | CMS 19      
Internal credit ratings systems      
Financial assets (236) (16)  
Loans and advances to customers | Commercial Banking | Impairment | CMS 20-23      
Internal credit ratings systems      
Financial assets (1,509) (941)  
Loans and advances to customers | Commercial Banking | Impairment | CMS      
Internal credit ratings systems      
Financial assets (2,571) (1,265)  
Loans and advances to customers | Commercial Banking | Impairment | 12-month expected credit losses      
Internal credit ratings systems      
Financial assets (270) (96)  
Loans and advances to customers | Commercial Banking | Impairment | 12-month expected credit losses | CMS 1-10      
Internal credit ratings systems      
Financial assets (21) (33)  
Loans and advances to customers | Commercial Banking | Impairment | 12-month expected credit losses | CMS 11-14      
Internal credit ratings systems      
Financial assets (131) (50)  
Loans and advances to customers | Commercial Banking | Impairment | 12-month expected credit losses | CMS 15-18      
Internal credit ratings systems      
Financial assets (118) (13)  
Loans and advances to customers | Commercial Banking | Impairment | 12-month expected credit losses | CMS      
Internal credit ratings systems      
Financial assets (270) (96)  
Loans and advances to customers | Commercial Banking | Impairment | Lifetime expected credit losses | Not credit impaired      
Internal credit ratings systems      
Financial assets (792) (228)  
Loans and advances to customers | Commercial Banking | Impairment | Lifetime expected credit losses | Not credit impaired | CMS 1-10      
Internal credit ratings systems      
Financial assets (2) (1)  
Loans and advances to customers | Commercial Banking | Impairment | Lifetime expected credit losses | Not credit impaired | CMS 11-14      
Internal credit ratings systems      
Financial assets (164) (37)  
Loans and advances to customers | Commercial Banking | Impairment | Lifetime expected credit losses | Not credit impaired | CMS 15-18      
Internal credit ratings systems      
Financial assets (390) (174)  
Loans and advances to customers | Commercial Banking | Impairment | Lifetime expected credit losses | Not credit impaired | CMS 19      
Internal credit ratings systems      
Financial assets (236) (16)  
Loans and advances to customers | Commercial Banking | Impairment | Lifetime expected credit losses | Not credit impaired | CMS      
Internal credit ratings systems      
Financial assets (792) (228)  
Loans and advances to customers | Commercial Banking | Impairment | Lifetime expected credit losses | Credit-impaired after purchase or origination      
Internal credit ratings systems      
Financial assets (1,509) (941)  
Loans and advances to customers | Commercial Banking | Impairment | Lifetime expected credit losses | Credit-impaired after purchase or origination | CMS 20-23      
Internal credit ratings systems      
Financial assets (1,509) (941)  
Loans and advances to customers | Commercial Banking | Impairment | Lifetime expected credit losses | Credit-impaired after purchase or origination | CMS      
Internal credit ratings systems      
Financial assets £ (1,509) £ (941)  
Loans and advances to customers | Other | CMS 1-10 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 0.00% 0.00%  
Expected credit loss PD (as a percent) 0.00% 0.00%  
Loans and advances to customers | Other | CMS 1-10 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 0.50% 0.50%  
Expected credit loss PD (as a percent) 0.50% 0.50%  
Loans and advances to customers | Other | CMS 11-14 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 0.51% 0.51%  
Expected credit loss PD (as a percent) 0.51% 0.51%  
Loans and advances to customers | Other | CMS 11-14 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 3.00% 3.00%  
Expected credit loss PD (as a percent) 3.00% 3.00%  
Loans and advances to customers | Other | CMS 15-18 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 3.01% 3.01%  
Expected credit loss PD (as a percent) 3.01% 3.01%  
Loans and advances to customers | Other | CMS 15-18 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 20.00% 20.00%  
Expected credit loss PD (as a percent) 20.00% 20.00%  
Loans and advances to customers | Other | CMS 19 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 20.01% 20.01%  
Expected credit loss PD (as a percent) 20.01% 20.01%  
Loans and advances to customers | Other | CMS 19 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 99.99% 99.99%  
Expected credit loss PD (as a percent) 99.99% 99.99%  
Loans and advances to customers | Other | CMS 20-23      
Internal credit ratings systems      
Regulatory PD (as a percent) 100.00% 100.00%  
Expected credit loss PD (as a percent) 100.00% 100.00%  
Loans and advances to customers | Other | RMS 1-6 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 0.00% 0.00%  
Expected credit loss PD (as a percent) 0.00% 0.00%  
Loans and advances to customers | Other | RMS 1-6 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 4.50% 4.50%  
Expected credit loss PD (as a percent) 4.50% 4.50%  
Loans and advances to customers | Other | RMS 7-9 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 4.51% 4.51%  
Expected credit loss PD (as a percent) 4.51% 4.51%  
Loans and advances to customers | Other | RMS 7-9 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 14.00% 14.00%  
Expected credit loss PD (as a percent) 14.00% 14.00%  
Loans and advances to customers | Other | RMS 10 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 14.01% 14.01%  
Expected credit loss PD (as a percent) 14.01% 14.01%  
Loans and advances to customers | Other | RMS 10 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 20.00% 20.00%  
Expected credit loss PD (as a percent) 20.00% 20.00%  
Loans and advances to customers | Other | RMS 11-13 | Minimum      
Internal credit ratings systems      
Regulatory PD (as a percent) 20.01% 20.01%  
Expected credit loss PD (as a percent) 20.01% 20.01%  
Loans and advances to customers | Other | RMS 11-13 | Maximum      
Internal credit ratings systems      
Regulatory PD (as a percent) 99.99% 99.99%  
Expected credit loss PD (as a percent) 99.99% 99.99%  
Loans and advances to customers | Other | RMS 14      
Internal credit ratings systems      
Regulatory PD (as a percent) 100.00% 100.00%  
Expected credit loss PD (as a percent) 100.00% 100.00%  
Loans and advances to customers | Other | Gross carrying amount      
Internal credit ratings systems      
Financial assets £ 64,652 £ 57,266  
Loans and advances to customers | Other | Gross carrying amount | CMS 1-10      
Internal credit ratings systems      
Financial assets 63,773 56,356  
Loans and advances to customers | Other | Gross carrying amount | CMS      
Internal credit ratings systems      
Financial assets 63,773 56,356  
Loans and advances to customers | Other | Gross carrying amount | RMS 1-6      
Internal credit ratings systems      
Financial assets 788 786  
Loans and advances to customers | Other | Gross carrying amount | RMS 7-9      
Internal credit ratings systems      
Financial assets   40  
Loans and advances to customers | Other | Gross carrying amount | RMS 14      
Internal credit ratings systems      
Financial assets 91 84  
Loans and advances to customers | Other | Gross carrying amount | RMS      
Internal credit ratings systems      
Financial assets 879 910  
Loans and advances to customers | Other | Gross carrying amount | 12-month expected credit losses      
Internal credit ratings systems      
Financial assets 64,538 57,150  
Loans and advances to customers | Other | Gross carrying amount | 12-month expected credit losses | CMS 1-10      
Internal credit ratings systems      
Financial assets 63,773 56,356  
Loans and advances to customers | Other | Gross carrying amount | 12-month expected credit losses | CMS      
Internal credit ratings systems      
Financial assets 63,773 56,356  
Loans and advances to customers | Other | Gross carrying amount | 12-month expected credit losses | RMS 1-6      
Internal credit ratings systems      
Financial assets 765 754  
Loans and advances to customers | Other | Gross carrying amount | 12-month expected credit losses | RMS 7-9      
Internal credit ratings systems      
Financial assets   40  
Loans and advances to customers | Other | Gross carrying amount | 12-month expected credit losses | RMS      
Internal credit ratings systems      
Financial assets 765 794  
Loans and advances to customers | Other | Gross carrying amount | Lifetime expected credit losses | Not credit impaired      
Internal credit ratings systems      
Financial assets 23 32  
Loans and advances to customers | Other | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | RMS 1-6      
Internal credit ratings systems      
Financial assets 23 32  
Loans and advances to customers | Other | Gross carrying amount | Lifetime expected credit losses | Not credit impaired | RMS      
Internal credit ratings systems      
Financial assets 23 32  
Loans and advances to customers | Other | Gross carrying amount | Lifetime expected credit losses | Credit-impaired after purchase or origination      
Internal credit ratings systems      
Financial assets 91 84  
Loans and advances to customers | Other | Gross carrying amount | Lifetime expected credit losses | Credit-impaired after purchase or origination | RMS 14      
Internal credit ratings systems      
Financial assets 91 84  
Loans and advances to customers | Other | Gross carrying amount | Lifetime expected credit losses | Credit-impaired after purchase or origination | RMS      
Internal credit ratings systems      
Financial assets 91 84  
Loans and advances to customers | Other | Impairment      
Internal credit ratings systems      
Financial assets (231) (33)  
Loans and advances to customers | Other | Impairment | CMS 1-10      
Internal credit ratings systems      
Financial assets   (10)  
Loans and advances to customers | Other | Impairment | CMS      
Internal credit ratings systems      
Financial assets   (10)  
Loans and advances to customers | Other | Impairment | RMS 1-6      
Internal credit ratings systems      
Financial assets (12) (7)  
Loans and advances to customers | Other | Impairment | RMS 14      
Internal credit ratings systems      
Financial assets (19) (16)  
Loans and advances to customers | Other | Impairment | RMS      
Internal credit ratings systems      
Financial assets (31) (23)  
Loans and advances to customers | Other | Impairment | 12-month expected credit losses      
Internal credit ratings systems      
Financial assets (212) (16)  
Loans and advances to customers | Other | Impairment | 12-month expected credit losses | CMS 1-10      
Internal credit ratings systems      
Financial assets   (10)  
Loans and advances to customers | Other | Impairment | 12-month expected credit losses | CMS      
Internal credit ratings systems      
Financial assets   (10)  
Loans and advances to customers | Other | Impairment | 12-month expected credit losses | RMS 1-6      
Internal credit ratings systems      
Financial assets (12) (6)  
Loans and advances to customers | Other | Impairment | 12-month expected credit losses | RMS      
Internal credit ratings systems      
Financial assets (12) (6)  
Loans and advances to customers | Other | Impairment | Lifetime expected credit losses | Not credit impaired      
Internal credit ratings systems      
Financial assets   (1)  
Loans and advances to customers | Other | Impairment | Lifetime expected credit losses | Not credit impaired | RMS 1-6      
Internal credit ratings systems      
Financial assets   (1)  
Loans and advances to customers | Other | Impairment | Lifetime expected credit losses | Not credit impaired | RMS      
Internal credit ratings systems      
Financial assets   (1)  
Loans and advances to customers | Other | Impairment | Lifetime expected credit losses | Credit-impaired after purchase or origination      
Internal credit ratings systems      
Financial assets (19) (16)  
Loans and advances to customers | Other | Impairment | Lifetime expected credit losses | Credit-impaired after purchase or origination | RMS 14      
Internal credit ratings systems      
Financial assets (19) (16)  
Loans and advances to customers | Other | Impairment | Lifetime expected credit losses | Credit-impaired after purchase or origination | RMS      
Internal credit ratings systems      
Financial assets (19) £ (16)  
Loans and advances to customers | Central adjustment to severe scenario | Impairment      
Internal credit ratings systems      
Financial assets (200)    
Loans and advances to customers | Central adjustment to severe scenario | Impairment | 12-month expected credit losses      
Internal credit ratings systems      
Financial assets £ (200)