Regulatory and Capital Adequacy - Capital Ratios Under Basel III (Details) $ in Millions	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Common equity Tier 1 capital:
Common equity Tier 1 capital	$ 43,501	$ 40,736
Common equity Tier 1 capital, capital ratio	0.131	0.137
Common equity Tier 1 capital, minimum capital adequacy	0.045	0.045
Tier 1 risk-based capital:
Tier 1 capital	$ 48,346	$ 45,583
Tier 1 capital, capital ratio	0.145	0.153
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	$ 56,089	$ 52,788
Total capital, capital ratio	0.169	0.177
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	$ 48,346	$ 45,583
Tier 1 leverage, capital ratio	0.116	0.112
Tier 1 leverage, minimum capital adequacy	0.040	0.040
Supplementary leverage:
Tier one capital under BaselIII standardized approach	$ 48,346	$ 45,583
Basel III Supplementary Leverage Ratio	0.099	0.107
Supplementary leverage	0.030	0.030
Regulatory Capital Adjustments, Net of Tax, CECL Transition	$ 2,400
Common Equity Tier One Capital Ratio, Excluding CECL Transition Adjustment	12.40%
Common Equity Tier One Capital Ratio, CECL Transition	13.10%
Capital One Bank (USA), N.A.
Common equity Tier 1 capital:
Common equity Tier 1 capital	$ 17,392	$ 19,924
Common equity Tier 1 capital, capital ratio	0.165	0.215
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	$ 17,392	$ 19,924
Tier 1 capital, capital ratio	0.165	0.215
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	$ 19,047	$ 21,708
Total capital, capital ratio	0.180	0.234
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	$ 17,392	$ 19,924
Tier 1 leverage, capital ratio	0.149	0.183
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	$ 17,392	$ 19,924
Basel III Supplementary Leverage Ratio	0.120	0.147
Supplementary leverage	0.030	0.030
Capital One, N.A.
Common equity Tier 1 capital:
Common equity Tier 1 capital	$ 26,699	$ 26,671
Common equity Tier 1 capital, capital ratio	0.111	0.124
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	$ 26,699	$ 26,671
Tier 1 capital, capital ratio	0.111	0.124
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	$ 29,449	$ 29,369
Total capital, capital ratio	0.122	0.137
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	$ 26,699	$ 26,671
Tier 1 leverage, capital ratio	0.074	0.076
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	$ 26,699	$ 26,671
Basel III Supplementary Leverage Ratio	0.066	0.069
Supplementary leverage	0.030	0.030