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Allowance for Credit Losses - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2022 | Mar. 31, 2022
Receivables [Abstract] |  |
Initial PCD ACL |  | $ 284.0
Initial Non-PCD Provision |  | 454.0
Commercial |  |
Receivables [Abstract] |  |
Initial PCD ACL |  | 270.0
Initial Non-PCD Provision |  | 432.0
Financing Receivable, Allowance for Credit Loss [Line Items] |  |
Allowance for credit loss, period increase (decrease) | $ 13.5 |
Consumer |  |
Receivables [Abstract] |  |
Initial PCD ACL |  | 14.0
Initial Non-PCD Provision |  | $ 22.0
Financing Receivable, Allowance for Credit Loss [Line Items] |  |
Allowance for credit loss, period increase (decrease) | $ (18.7) |

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- Definition Financing Receivable, Credit Loss, Expense + References No definition available. + Details
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Namespace Prefix: | fcnca_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of increase (decrease) in allowance for credit loss on financing receivable. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524 Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447 + Details
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Balance Type: | na
Period Type: | duration

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- Definition Amount of allowance for credit loss at acquisition date of financing receivable purchased with credit deterioration. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 19 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919265-210447 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
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Balance Type: |
Period Type: |