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FDIC Shared-Loss Payable (Narrative) (Details) - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019
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FDIC Shared-Loss Receivable [Abstract] |  |
Loans and Leases Receivable, Gross, Carrying Amount, Covered | $ 36,200 |
FDIC shared-loss payable | $ 15,313 | $ 112,395

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- Definition FDIC Shared-Loss Receivable [Abstract] + References No definition available. + Details
Name: | fcnca_FDICSharedLossReceivableAbstract
Namespace Prefix: | fcnca_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Payable to FDIC for Loss Share Agreements + References No definition available. + Details
Name: | fcnca_PayableToFdicForLossShareAgreements
Namespace Prefix: | fcnca_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Aggregate gross carrying amount of loans and leases receivable covered by loss sharing agreements. + References No definition available. + Details
Name: | us-gaap_LoansAndLeasesReceivableGrossCarryingAmountCovered
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant