Line of Credit Facility, Additional Borrowing Capacity

No definition available.

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

- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ References

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+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of assets pledged to secure a debt instrument.

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Balance Type:	debit
Period Type:	instant

- Definition

Face (par) amount of debt instrument at time of issuance.

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+ Details

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- 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

No definition available.

+ Details

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

- Definition

Weighted average interest rate of debt outstanding.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Gross amount of debt extinguished.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

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+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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+ Details

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

- Definition

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

+ Details

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

- Definition

Amount, after the effects of master netting arrangements, of funds outstanding borrowed in the form of a security repurchase agreement between the entity and another party for the sale and repurchase of identical or substantially the same securities at a date certain for a specified price. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

+ References

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+ Details

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Balance Type:	credit
Period Type:	instant

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PledgedStatusAxis=us-gaap_AssetPledgedAsCollateralMember
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Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CommercialMortgageBackedSecuritiesMember
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Balance Type:
Period Type:

- Details

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- Details

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Balance Type:
Period Type:

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

- Details

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- Details

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Balance Type:
Period Type:

- Details

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Namespace Prefix:
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Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=fcnca_OtherSecuredDebtMember
Namespace Prefix:
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Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SecuredDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LineOfCreditFacilityAxis=fcnca_FederalReserveBankOfRichmondMember
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Balance Type:
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- Details

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Borrowings - Narrative (Details) - USD ($) $ in Millions		12 Months Ended
Borrowings - Narrative (Details) - USD ($) $ in Millions	Feb. 24, 2022	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Debt Instrument [Line Items]
Securities sold under customer repurchase agreements		$ 436.0	$ 589.0
Gain on extinguishment of debt		7.0	0.0	$ 0.0
Debt instrument, collateral amount		29,200.0
Line of credit facility, remaining borrowing capacity		100.0
Government agency \| Asset Pledged as Collateral
Debt Instrument [Line Items]
Securities sold under customer repurchase agreements			508.0
Commercial mortgage-backed securities \| Asset Pledged as Collateral
Debt Instrument [Line Items]
Securities sold under customer repurchase agreements			81.0
Securities Assets \| Asset Pledged as Collateral
Debt Instrument [Line Items]
Securities sold under customer repurchase agreements		496.0	619.0
Trust Preferred Securities Subject to Mandatory Redemption
Debt Instrument [Line Items]
Long-term debt, gross		128.0	128.0
Fixed-to-Floating subordinated notes at 3.375% \| Subordinated Debt
Debt Instrument [Line Items]
Long-term debt, gross		$ 350.0
Debt instrument, stated interest rate		3.375%
Fixed subordinated notes at 6.125% \| Subordinated Debt
Debt Instrument [Line Items]
Long-term debt, gross		$ 400.0
Debt instrument, stated interest rate		6.125%
Fixed-to-Fixed subordinated notes at 4.125% \| Subordinated Debt
Debt Instrument [Line Items]
Long-term debt, gross		$ 100.0
Debt instrument, stated interest rate		4.125%
Junior subordinated debentures \| Subordinated Debt
Debt Instrument [Line Items]
Long-term debt, gross		$ 132.0
Other secured financings \| Secured Debt
Debt Instrument [Line Items]
Debt instrument, collateral amount		18.0
Debt instrument, face amount		18.0
Other secured financings \| Secured Debt \| Federal Reserve Bank of Richmond
Debt Instrument [Line Items]
Debt instrument, collateral amount		5,700.0
Line of credit facility, additional borrowing capacity		4,200.0
Long-term line of credit		0.0	$ 0.0
Fixed rate senior unsecured notes \| Senior Notes
Debt Instrument [Line Items]
Long-term debt, gross		$ 866.0
Debt, weighted average interest rate		3.70%
Fixed rate senior unsecured notes assumed in business combination \| Senior Notes
Debt Instrument [Line Items]
Redemption of debt	$ 2,900.0
Gain on extinguishment of debt	$ 6.0
Notes Payable To FHLB \| Secured Debt \| Asset Pledged as Collateral
Debt Instrument [Line Items]
Debt instrument, collateral amount		$ 23,500.0