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Regulatory and Capital Adequacy - Schedule of Comparison of Capital Ratios (Details) - USD ($)
$ in Millions
Dec. 31, 2017
Dec. 31, 2016
Common equity Tier 1 capital:    
Common equity Tier 1 Capital $ 30,036 $ 28,803
Common equity Tier 1 capital, capital ratio 10.30% 10.10%
Common equity Tier 1 capital, minimum capital adequacy 4.50% 4.50%
Tier 1 risk-based capital:    
Tier 1 capital $ 34,396 $ 33,162
Tier 1 capital, capital ratio 11.80% 11.60%
Tier 1 capital, minimum capital adequacy 6.00% 6.00%
Tier 1 capital, well-capitalized 6.00% 6.00%
Total risk-based capital:    
Total capital $ 41,962 $ 40,817
Total capital, capital ratio 14.40% 14.30%
Total capital, minimum capital adequacy 8.00% 8.00%
Total capital, well-capitalized 10.00% 10.00%
Tier 1 leverage capital:    
Tier 1 leverage capital $ 34,396 $ 33,162
Tier 1 leverage, capital ratio 9.90% 9.90%
Tier 1 leverage, minimum capital adequacy 4.00% 4.00%
Tier one capital under BaselIII standardized approach $ 34,396 $ 33,162
Basel III Supplementary Leverage Ratio 8.40% 8.60%
Capital One Bank (USA), N.A.    
Common equity Tier 1 capital:    
Common equity Tier 1 Capital $ 14,791 $ 11,568
Common equity Tier 1 capital, capital ratio 14.30% 12.00%
Common equity Tier 1 capital, minimum capital adequacy 4.50% 4.50%
Common equity Tie 1 capital, well capitalized 6.50% 6.50%
Tier 1 risk-based capital:    
Tier 1 capital $ 14,791 $ 11,568
Tier 1 capital, capital ratio 14.30% 12.00%
Tier 1 capital, minimum capital adequacy 6.00% 6.00%
Tier 1 capital, well-capitalized 8.00% 8.00%
Total risk-based capital:    
Total capital $ 17,521 $ 14,230
Total capital, capital ratio 16.90% 14.80%
Total capital, minimum capital adequacy 8.00% 8.00%
Total capital, well-capitalized 10.00% 10.00%
Tier 1 leverage capital:    
Tier 1 leverage capital $ 14,791 $ 11,568
Tier 1 leverage, capital ratio 12.70% 10.80%
Tier 1 leverage, minimum capital adequacy 4.00% 4.00%
Tier 1 leverage, well-capitalized 5.00% 5.00%
Tier one capital under BaselIII standardized approach $ 14,791 $ 11,568
Basel III Supplementary Leverage Ratio 10.40% 8.90%
Capital One, N.A.    
Common equity Tier 1 capital:    
Common equity Tier 1 Capital $ 23,771 $ 20,670
Common equity Tier 1 capital, capital ratio 12.20% 10.60%
Common equity Tier 1 capital, minimum capital adequacy 4.50% 4.50%
Common equity Tie 1 capital, well capitalized 6.50% 6.50%
Tier 1 risk-based capital:    
Tier 1 capital $ 23,771 $ 20,670
Tier 1 capital, capital ratio 12.20% 10.60%
Tier 1 capital, minimum capital adequacy 6.00% 6.00%
Tier 1 capital, well-capitalized 8.00% 8.00%
Total risk-based capital:    
Total capital $ 26,214 $ 23,117
Total capital, capital ratio 13.40% 11.80%
Total capital, minimum capital adequacy 8.00% 8.00%
Total capital, well-capitalized 10.00% 10.00%
Tier 1 leverage capital:    
Tier 1 leverage capital $ 23,771 $ 20,670
Tier 1 leverage, capital ratio 8.60% 7.70%
Tier 1 leverage, minimum capital adequacy 4.00% 4.00%
Tier 1 leverage, well-capitalized 5.00% 5.00%
Tier one capital under BaselIII standardized approach $ 23,771 $ 20,670
Basel III Supplementary Leverage Ratio 7.70% 6.90%