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FDIC Shared-Loss Payable (Narrative) (Details) $ in Thousands | 3 Months Ended
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Mar. 31, 2021 USD ($)
FDIC Shared-Loss Receivable [Abstract] |
Payment made to the FDIC to settle shared-loss agreement | $ 16,103

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- Definition FDIC shared-loss payable adjustments related to changes in assumptions. + References No definition available. + Details
Name: | fcnca_FDICSharedLossPayableAdjustments
Namespace Prefix: | fcnca_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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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