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FDIC Shared-Loss Payable - Narrative (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
FDIC Shared-Loss Receivable [Abstract] |  |  |
FDIC shared-loss payable | $ 0 | $ 15,601 | $ 112,395
Payment made to the FDIC to settle shared-loss agreement | $ (16,103) | $ (99,468) |

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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

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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