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v3.20.4

FDIC Shared-Loss Payable (Changes in Receivable from FDIC) (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2020 | Dec. 31, 2020 | Dec. 31, 2019
FDIC Indemnification Asset [Roll Forward] |  |  |
Beginning balance | $ 112,395 | $ 112,395 | $ 105,618
Accretion |  | 2,674 | 6,777
Payment made to the FDIC to settle shared-loss agreement | $ 99,500 | (99,468) | 0
Ending balance |  | $ 15,601 | $ 112,395

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- Definition Amount of accretion expense for FDIC shared-loss payable. + References No definition available. + Details
Name: | fcnca_AccretionOfFDICSharedLossPayable
Namespace Prefix: | fcnca_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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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 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 A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period. + References No definition available. + Details
Name: | us-gaap_FDICIndemnificationAssetRollForward
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration