ALLOWANCE FOR LOAN AND LEASE LOSSES - Schedule of Components of Credit Losses (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Receivables [Abstract]
Provision for loan and lease losses	$ 214	$ 123	$ 473	$ 311
Benefit for off-balance sheet credit exposure	(23)	(6)	(13)	(35)
Provision for credit losses	$ 191	$ 117	$ 460	$ 276

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