ALLOWANCE FOR LOAN AND LEASE LOSSES - Schedule of Components of Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Receivables [Abstract]
Provision for loan and lease losses	$ 111	$ 95	$ 259	$ 188
Provision (benefit) for off-balance sheet credit exposure	4	0	10	(29)
Provision for credit losses	$ 115	$ 95	$ 269	$ 159

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