Regulatory and Capital Adequacy Regulatory and Capital Adequacy - Additional Information (Details) - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Basel III Transition Provisions, Inclusion of AOCI and Intangible Asset Adjustments			60.00%	40.00%
Forecast
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Basel III Transition Provisions, Inclusion of AOCI and Intangible Asset Adjustments	100.00%	80.00%
Capital One Bank (USA), N.A.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Statutory Accounting Practices, Statutory Amount Available for Dividend Payments			$ 3.9
Capital One, N.A.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Statutory Accounting Practices, Statutory Amount Available for Dividend Payments			$ 1.0