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v3.23.1

Other Assets - Narrative (Details) $ in Millions | Mar. 27, 2023 USD ($)
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SVB |
Business Acquisition [Line Items] |
Business combination, miscellaneous assets | $ 2,180

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- Definition Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Miscellaneous Assets + References No definition available. + Details
Name: | fcnca_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedMiscellaneousAssets
Namespace Prefix: | fcnca_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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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
Name: | us-gaap_BusinessAcquisitionLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_BusinessAcquisitionAxis=fcnca_SiliconValleyBridgeBankNAMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |