Capital One Financial Corporation (Parent Company Only) - Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets:
Cash and cash equivalents	$ 40,509	$ 13,407
Securities available for sale (amortized cost of $97.6 billion and allowance for credit losses of $1 million as of December 31, 2020)	100,445	79,213
Other assets	21,205	17,613
Total assets	421,602	390,365
Liabilities:
Senior and subordinated notes	27,382	30,472
Accrued expenses and other liabilities	15,065	13,521
Total liabilities	361,398	332,354
Stockholders’ equity:
Total stockholders’ equity	60,204	58,011	$ 51,668	$ 48,730
Total liabilities and stockholders’ equity	421,602	390,365
Parent Company
Assets:
Cash and cash equivalents	12,976	13,050
Investments in subsidiaries	62,066	61,626
Loans to subsidiaries	5,924	3,905
Securities available for sale (amortized cost of $97.6 billion and allowance for credit losses of $1 million as of December 31, 2020)	622	738
Other assets	1,473	1,017
Total assets	83,061	80,336
Liabilities:
Senior and subordinated notes	22,037	22,080
Accrued expenses and other liabilities	820	245
Total liabilities	22,857	22,325
Stockholders’ equity:
Total stockholders’ equity	60,204	58,011
Total liabilities and stockholders’ equity	$ 83,061	$ 80,336