Regulatory and Capital Adequacy - Schedule of Comparison of Capital Ratios (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Common equity Tier 1 capital:
Common equity Tier 1 capital	$ 33,071	$ 30,036
Common equity Tier 1 capital, capital ratio	11.20%	10.30%
Common equity Tier 1 capital, minimum capital adequacy	4.50%	4.50%
Tier 1 risk-based capital:
Tier 1 capital	$ 37,431	$ 34,396
Tier 1 capital, capital ratio	12.70%	11.80%
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	$ 44,645	$ 41,962
Total capital, capital ratio	15.10%	14.40%
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	$ 37,431	$ 34,396
Tier 1 leverage, capital ratio	10.70%	9.90%
Tier 1 leverage, minimum capital adequacy	4.00%	4.00%
Supplementary leverage:
Tier one capital under BaselIII standardized approach	$ 37,431	$ 34,396
Basel III Supplementary Leverage Ratio	9.00%	8.40%
Minimum Capital Adequacy for Supplementary Leverage Ratio	3.00%
Basel III Transition Provisions, Inclusion of AOCI and Intangible Asset Adjustments	100.00%	80.00%
Capital One Bank (USA), N.A.
Common equity Tier 1 capital:
Common equity Tier 1 capital	$ 16,378	$ 14,791
Common equity Tier 1 capital, capital ratio	15.30%	14.30%
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	$ 16,378	$ 14,791
Tier 1 capital, capital ratio	15.30%	14.30%
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	$ 18,788	$ 17,521
Total capital, capital ratio	17.60%	16.90%
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	$ 16,378	$ 14,791
Tier 1 leverage, capital ratio	14.00%	12.70%
Tier 1 leverage, minimum capital adequacy	4.00%	4.00%
Tier 1 leverage, well-capitalized	5.00%	5.00%
Supplementary leverage:
Tier one capital under BaselIII standardized approach	$ 16,378	$ 14,791
Basel III Supplementary Leverage Ratio	11.50%	10.40%
Minimum Capital Adequacy for Supplementary Leverage Ratio	3.00%
Capital One, N.A.
Common equity Tier 1 capital:
Common equity Tier 1 capital	$ 25,637	$ 23,771
Common equity Tier 1 capital, capital ratio	13.00%	12.20%
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	$ 25,637	$ 23,771
Tier 1 capital, capital ratio	13.00%	12.20%
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	$ 27,912	$ 26,214
Total capital, capital ratio	14.20%	13.40%
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	$ 25,637	$ 23,771
Tier 1 leverage, capital ratio	9.10%	8.60%
Tier 1 leverage, minimum capital adequacy	4.00%	4.00%
Tier 1 leverage, well-capitalized	5.00%	5.00%
Supplementary leverage:
Tier one capital under BaselIII standardized approach	$ 25,637	$ 23,771
Basel III Supplementary Leverage Ratio	8.00%	7.70%
Minimum Capital Adequacy for Supplementary Leverage Ratio	3.00%

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