LOANS AND LEASES - Schedule of Financial Effects of Loans Modified (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Term Extension (in Months)	13 months	14 months	15 months	19 months
Weighted Average Interest Rate Reduction	0.71%	2.19%	0.95%	1.07%
Weighted Average Payment Delay (in Months)	11 months	6 months	10 months	14 months
Financing receivable, excluding accrued interest, modified, amortized cost of loans (less than)	$ 1			$ 1
Commercial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Term Extension (in Months)	13 months	14 months	15 months	18 months
Weighted Average Interest Rate Reduction	0.65%	2.03%	0.91%	0.96%
Weighted Average Payment Delay (in Months)	11 months	6 months	10 months	14 months
Commercial \| Commercial construction
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Term Extension (in Months)	8 months	36 months	7 months	15 months
Weighted Average Interest Rate Reduction	0.00%	0.60%	0.00%	0.60%
Weighted Average Payment Delay (in Months)	7 months	0 months	6 months	0 months
Commercial \| Owner occupied commercial mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Term Extension (in Months)	13 months	17 months	11 months	28 months
Weighted Average Interest Rate Reduction	0.00%	1.82%	1.93%	1.67%
Weighted Average Payment Delay (in Months)	6 months	6 months	5 months	18 months
Commercial \| Non-owner occupied commercial mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Term Extension (in Months)	9 months	7 months	17 months	20 months
Weighted Average Interest Rate Reduction	0.52%	0.00%	0.56%	0.00%
Weighted Average Payment Delay (in Months)	36 months	6 months	14 months	40 months
Commercial \| Commercial and industrial
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Term Extension (in Months)	15 months	15 months	14 months	15 months
Weighted Average Interest Rate Reduction	1.05%	2.61%	1.31%	0.71%
Weighted Average Payment Delay (in Months)	12 months	6 months	12 months	12 months
Commercial \| Leases
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Term Extension (in Months)	60 months	11 months	60 months	11 months
Weighted Average Interest Rate Reduction		0.00%	0.00%	0.00%
Weighted Average Payment Delay (in Months)	0 months	0 months	0 months	6 months
Commercial \| Investor dependent
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Term Extension (in Months)	11 months	9 months	9 months	11 months
Weighted Average Interest Rate Reduction	0.00%	0.00%	0.00%	2.75%
Weighted Average Payment Delay (in Months)	6 months	6 months	7 months	8 months
Consumer
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Term Extension (in Months)	24 months	52 months	16 months	66 months
Weighted Average Interest Rate Reduction	3.01%	3.29%	2.39%	3.41%
Weighted Average Payment Delay (in Months)	8 months	11 months	6 months	11 months
Consumer \| Residential mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Term Extension (in Months)	21 months	47 months	14 months	70 months
Weighted Average Interest Rate Reduction	1.52%	2.07%	1.21%	2.09%
Weighted Average Payment Delay (in Months)	8 months	11 months	6 months	11 months
Consumer \| Revolving mortgage
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Term Extension (in Months)	58 months	60 months	48 months	60 months
Weighted Average Interest Rate Reduction	4.36%	5.42%	4.04%	4.73%
Weighted Average Payment Delay (in Months)	0 months	0 months	5 months	0 months
Consumer \| Consumer auto
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Term Extension (in Months)	18 months	31 months	19 months	29 months
Weighted Average Interest Rate Reduction	0.00%	0.00%	0.00%	0.26%
Weighted Average Payment Delay (in Months)	0 months	0 months	0 months	0 months
Consumer \| Consumer other
Accounts, Notes, Loans and Financing Receivable [Line Items]
Weighted Average Term Extension (in Months)	60 months	60 months	60 months	57 months
Weighted Average Interest Rate Reduction	11.41%	10.54%	9.93%	9.65%
Weighted Average Payment Delay (in Months)	0 months	0 months	0 months	0 months

Financing Receivable, Excluding Accrued Interest, Modified, Amortized Cost of loans

No definition available.

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

- Definition

Financing Receivable, Modified, Weighted Average Payment Delay

+ References

No definition available.

+ Details

Name:	fcnca_FinancingReceivableModifiedWeightedAveragePaymentDelay
Namespace Prefix:	fcnca_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
-SubTopic 20
-Name Accounting Standards Codification
-Section 40
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481628/310-20-40-7

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481990/310-10-45-2

+ Details

Name:	us-gaap_AccountsNotesAndLoansReceivableLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of decrease in weighted average contractual interest rate of financing receivable modified for debtor experiencing financial difficulty.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 12A
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481933/310-10-55-12A

+ Details

Name:	us-gaap_FinancingReceivableModifiedWeightedAverageInterestRateDecreaseFromModification
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Period of increase in weighted average maturity of financing receivable modified for debtor experiencing financial difficulty, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModifiedWeightedAverageTermIncreaseFromModification
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fcnca_OwnerOccupiedCommercialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fcnca_NonOwnerOccupiedCommercialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fcnca_CommercialAndIndustrialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fcnca_FinancingReceivableLeasesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fcnca_InvestorDependentEarlyStageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ResidentialMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fcnca_RevolvingMortgageMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_AutomobileLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fcnca_ConsumerLoanOtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: