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ALLOWANCE FOR LOAN AND LEASE LOSSES - Schedule of Components of Credit Losses (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2025 | Mar. 31, 2024
Receivables [Abstract] |  |
Provision for loan and lease losses | $ 148 | $ 93
Provision (benefit) for off-balance sheet credit exposure | 6 | (29)
Provision for credit losses | $ 154 | $ 64

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- Definition Credit Loss Expense (Reversal) + References No definition available. + Details
Name: | fcnca_CreditLossExpenseReversal
Namespace Prefix: | fcnca_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Financing Receivable, Credit Loss, Expense + References No definition available. + Details
Name: | fcnca_FinancingReceivableCreditLossExpense
Namespace Prefix: | fcnca_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Off-Balance-Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal), Excluding Amount Acquired In Acquisition + References No definition available. + Details
Name: | fcnca_OffBalanceSheetCreditLossLiabilityCreditLossExpenseReversalExcludingAmountAcquiredInAcquisition
Namespace Prefix: | fcnca_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_ReceivablesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration