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Regulatory and Capital Adequacy - Additional Information (Details) - USD ($) $ in Billions | Dec. 31, 2020 | Dec. 31, 2019
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Cash Reserves, Federal Reserve Bank |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Reserve requirement |  | $ 1.7
Capital One Bank (USA), N.A. |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Funds available for dividend payments | $ 4.0 |
Capital One, N.A. |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Funds available for dividend payments | $ 1.8 |

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- Definition Cash deposited in a special reserve account for the exclusive benefit of customers pursuant to SEC Regulations. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=116631393&loc=SL116631396-227033 + Details
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- Definition Amount of dividends available for payment after computation of statutory restrictions on capital surplus. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08.(e)) -URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690 + Details
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