Capital One Financial Corporation (Parent Company Only) - Statements of Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating activities:
Net income	$ 2,714	$ 5,546	$ 6,015
Adjustments to reconcile net income to net cash from operating activities:
Net cash from operating activities	16,699	16,639	12,978
Investing activities:
Net cash from investing activities	(14,841)	(22,998)	(15,618)
Common stock:
Net proceeds from issuances	241	199	175
Dividends paid	(460)	(753)	(773)
Preferred stock:
Net proceeds from issuances	1,330	1,462	0
Dividends paid	(280)	(282)	(265)
Redemptions	(1,375)	(1,000)	0
Purchases of treasury stock	(393)	(1,481)	(2,284)
Proceeds from share-based payment activities	62	17	38
Net cash from financing activities	25,164	6,619	1,777
Changes in cash and cash equivalents	27,022	260	(863)
Cash, cash equivalents and restricted cash for securitization investors, beginning of the period	13,749	13,489	14,352
Cash, cash equivalents and restricted cash for securitization investors, end of the period	40,771	13,749	13,489
Parent Company
Operating activities:
Net income	2,714	5,546	6,015
Adjustments to reconcile net income to net cash from operating activities:
Equity in undistributed earnings of subsidiaries	(102)	(2,569)	(3,554)
Other operating activities	1,217	216	(35)
Net cash from operating activities	3,829	3,193	2,426
Investing activities:
Changes in investments in subsidiaries	(217)	704	(577)
Proceeds from paydowns and maturities of securities available for sale	117	111	140
Changes in loans to subsidiaries	(2,019)	(1,302)	(2,055)
Net cash from investing activities	(2,119)	(487)	(2,492)
Financing activities:
Changes in borrowings from subsidiaries	0	0	38
Issuance of senior and subordinated notes	1,991	2,646	5,227
Maturities and paydowns of senior and subordinated notes	(2,900)	(750)	0
Common stock:
Net proceeds from issuances	241	199	175
Dividends paid	(460)	(753)	(773)
Preferred stock:
Net proceeds from issuances	1,330	1,462	0
Dividends paid	(280)	(282)	(265)
Redemptions	(1,375)	(1,000)	0
Purchases of treasury stock	(393)	(1,481)	(2,284)
Proceeds from share-based payment activities	62	17	38
Net cash from financing activities	(1,784)	58	2,156
Changes in cash and cash equivalents	(74)	2,764	2,090
Cash, cash equivalents and restricted cash for securitization investors, beginning of the period	13,050	10,286	8,196
Cash, cash equivalents and restricted cash for securitization investors, end of the period	12,976	13,050	10,286
Supplemental information:
Decrease in investment in subsidiaries	0	1,508	0
Decrease in borrowings from subsidiaries	$ 0	$ 1,671	$ 0