LOANS AND LEASES - Schedule of Loans Pledged as Collateral (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024	Mar. 31, 2024
FHLB of Atlanta
Pledged non-PCD loans	$ 141,358	$ 140,221	$ 135,370
FHLB of Atlanta \| Non-PCD \| Asset Pledged as Collateral
FHLB of Atlanta
Lendable collateral value of pledged non-PCD loans	17,992	17,873
Less: advances	0	0
Less: letters of credit	1,450	1,450
Available borrowing capacity	16,542	16,423
Pledged non-PCD loans	30,112	30,421
FRB \| Non-PCD \| Asset Pledged as Collateral
FHLB of Atlanta
Pledged non-PCD loans	6,432	6,309
FRB
Lendable collateral value of pledged non-PCD loans	5,612	5,475
Less: advances	0	0
Available borrowing capacity	5,612	5,475
FDIC \| Asset Pledged as Collateral
FHLB of Atlanta
Pledged non-PCD loans	39,173	41,040
FDIC
Lendable collateral value of pledged loans	39,415	41,282
Less: advances	0	0
Less: Purchase Money Note	35,991	35,991
Available borrowing capacity	$ 0	$ 5,291

Advances From Federal Deposit Insurance Corporation

No definition available.

Name:	fcnca_AdvancesFromFederalDepositInsuranceCorporation
Namespace Prefix:	fcnca_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Advances from Federal Reserve Bank

+ References

No definition available.

+ Details

Name:	fcnca_AdvancesFromFederalReserveBank
Namespace Prefix:	fcnca_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Federal Deposit Insurance Corporation

+ References

No definition available.

+ Details

Name:	fcnca_FederalDepositInsuranceCorporationAbstract
Namespace Prefix:	fcnca_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Federal Deposit Insurance Corporation, General Debt Obligations, Amount of Available, Unused Funds

+ References

No definition available.

+ Details

Name:	fcnca_FederalDepositInsuranceCorporationGeneralDebtObligationsAmountOfAvailableUnusedFunds
Namespace Prefix:	fcnca_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Federal Deposit Insurance Corporation, General Debt Obligations, Maximum Amount Available

+ References

No definition available.

+ Details

Name:	fcnca_FederalDepositInsuranceCorporationGeneralDebtObligationsMaximumAmountAvailable
Namespace Prefix:	fcnca_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Federal Reserve Bank

+ References

No definition available.

+ Details

Name:	fcnca_FederalReserveBankAbstract
Namespace Prefix:	fcnca_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Federal Reserve Bank, Advances, General Debt Obligations, Amount of Available, Unused Funds

+ References

No definition available.

+ Details

Name:	fcnca_FederalReserveBankAdvancesGeneralDebtObligationsAmountOfAvailableUnusedFunds
Namespace Prefix:	fcnca_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Federal Reserve Bank, Advances, General Debt Obligations, Maximum Amount Available

+ References

No definition available.

+ Details

Name:	fcnca_FederalReserveBankAdvancesGeneralDebtObligationsMaximumAmountAvailable
Namespace Prefix:	fcnca_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Letters Of Credit From Federal Home Loan Bank

+ References

No definition available.

+ Details

Name:	fcnca_LettersOfCreditFromFederalHomeLoanBank
Namespace Prefix:	fcnca_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Purchase Money Note From Federal Deposit Insurance Corporation

+ References

No definition available.

+ Details

Name:	fcnca_PurchaseMoneyNoteFromFederalDepositInsuranceCorporation
Namespace Prefix:	fcnca_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of borrowings as of the balance sheet date from the Federal Home Loan Bank, which are primarily used to cover shortages in the required reserve balance and liquidity shortages.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 470
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478878/942-470-45-1

+ Details

Name:	us-gaap_AdvancesFromFederalHomeLoanBanks
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of advances or credit lines available from the Federal Home Loan Bank (FHLB).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22)(b))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13)(3)(a))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 470
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478878/942-470-45-1

+ Details

Name:	us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresAmountOfAvailableUnusedFunds
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of maximum advances or credit lines available from the Federal Home Loan Bank (FHLB).

+ References

+ Details

Name:	us-gaap_FederalHomeLoanBankAdvancesGeneralDebtObligationsDisclosuresMaximumAmountAvailable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

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

- Definition

Amortized cost excluding accrued interest, before allowance for credit loss, of financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479344/326-20-45-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-3B

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-5

+ Details

Name:	us-gaap_FinancingReceivableExcludingAccruedInterestBeforeAllowanceForCreditLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	srt_CounterpartyNameAxis=fcnca_FHLBOfAtlantaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditLossStatusAxis=us-gaap_FinancialAssetAcquiredAndNoCreditDeteriorationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PledgedStatusAxis=us-gaap_AssetPledgedAsCollateralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=fcnca_FederalReserveBankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=fcnca_FederalDepositInsuranceCorporationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: