ALLOWANCE FOR LOAN AND LEASE LOSSES - Schedule of Components of Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended
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

Off-Balance-Sheet, Credit Loss, Liability, Credit Loss Expense (Reversal), Excluding Amount Acquired In Acquisition

No definition available.

Name:	fcnca_OffBalanceSheetCreditLossLiabilityCreditLossExpenseReversalExcludingAmountAcquiredInAcquisition
Namespace Prefix:	fcnca_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration