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Regulatory and Capital Adequacy - Capital Ratios Under Basel III (Details)
$ in Millions
Dec. 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Common equity Tier 1 capital:    
Common equity Tier 1 capital $ 40,736 $ 38,162
Common equity Tier 1 capital, capital ratio 0.137 0.122
Common equity Tier 1 capital, minimum capital adequacy 0.045 0.045
Tier 1 risk-based capital:    
Tier 1 capital $ 45,583 $ 43,015
Tier 1 capital, capital ratio 0.153 0.137
Tier 1 capital, minimum capital adequacy 0.060 0.060
Tier 1 capital, well-capitalized 0.060 0.060
Total risk-based capital:    
Total capital $ 52,788 $ 50,348
Total capital, capital ratio 0.177 0.161
Total capital, minimum capital adequacy 0.080 0.080
Total capital, well-capitalized 0.100 0.100
Tier 1 leverage capital:    
Tier 1 leverage capital $ 45,583 $ 43,015
Tier 1 leverage, capital ratio 0.112 0.117
Tier 1 leverage, minimum capital adequacy 0.040 0.040
Supplementary leverage:    
Tier one capital under BaselIII standardized approach $ 45,583 $ 43,015
Basel III Supplementary Leverage Ratio 0.107 0.099
Supplementary leverage 0.030 0.030
Capital One Bank (USA), N.A.    
Common equity Tier 1 capital:    
Common equity Tier 1 capital $ 19,924 $ 17,883
Common equity Tier 1 capital, capital ratio 0.215 0.161
Common equity Tier 1 capital, minimum capital adequacy 0.045 0.045
Common equity Tie 1 capital, well capitalized 0.065 0.065
Tier 1 risk-based capital:    
Tier 1 capital $ 19,924 $ 17,883
Tier 1 capital, capital ratio 0.215 0.161
Tier 1 capital, minimum capital adequacy 0.060 0.060
Tier 1 capital, well-capitalized 0.080 0.080
Total risk-based capital:    
Total capital $ 21,708 $ 20,109
Total capital, capital ratio 0.234 0.181
Total capital, minimum capital adequacy 0.080 0.080
Total capital, well-capitalized 0.100 0.100
Tier 1 leverage capital:    
Tier 1 leverage capital $ 19,924 $ 17,883
Tier 1 leverage, capital ratio 0.183 0.148
Tier 1 leverage, minimum capital adequacy 0.040 0.040
Tier 1 leverage, well-capitalized 0.050 0.050
Supplementary leverage:    
Tier one capital under BaselIII standardized approach $ 19,924 $ 17,883
Basel III Supplementary Leverage Ratio 0.147 0.121
Supplementary leverage 0.030 0.030
Capital One, N.A.    
Common equity Tier 1 capital:    
Common equity Tier 1 capital $ 26,671 $ 28,445
Common equity Tier 1 capital, capital ratio 0.124 0.134
Common equity Tier 1 capital, minimum capital adequacy 0.045 0.045
Common equity Tie 1 capital, well capitalized 0.065 0.065
Tier 1 risk-based capital:    
Tier 1 capital $ 26,671 $ 28,445
Tier 1 capital, capital ratio 0.124 0.134
Tier 1 capital, minimum capital adequacy 0.060 0.060
Tier 1 capital, well-capitalized 0.080 0.080
Total risk-based capital:    
Total capital $ 29,369 $ 30,852
Total capital, capital ratio 0.137 0.145
Total capital, minimum capital adequacy 0.080 0.080
Total capital, well-capitalized 0.100 0.100
Tier 1 leverage capital:    
Tier 1 leverage capital $ 26,671 $ 28,445
Tier 1 leverage, capital ratio 0.076 0.092
Tier 1 leverage, minimum capital adequacy 0.040 0.040
Tier 1 leverage, well-capitalized 0.050 0.050
Supplementary leverage:    
Tier one capital under BaselIII standardized approach $ 26,671 $ 28,445
Basel III Supplementary Leverage Ratio 0.069 0.082
Supplementary leverage 0.030 0.030