Estimated Fair Values (Fair Value Option) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Fair Value, Option, Quantitative Disclosures [Line Items]
Gains (Losses) from fair value changes on originated loans held for sale	$ 400	$ 2,400	$ (2,300)	$ 3,600
Loans Held For Sale
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value of originated loans held for sale	107,768		107,768		$ 124,837
Aggregate unpaid principle balance of originated loans held for sale	104,167		104,167		118,902
Difference between fair value and aggregate unpaid principle balance of originated loans held for sale	$ 3,601		$ 3,601		$ 5,935

Represents the aggregate unpaid principal balance of assets for which the fair value option was elected.

No definition available.

Name:	fcnca_AggregateUnpaidPrincipalBalanceOfItemsForWhichFairValueOptionWasElectedAssets
Namespace Prefix:	fcnca_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Represents the fair value of assets for which the fair value option was elected.

+ References

No definition available.

+ Details

Name:	fcnca_FairValueOfItemsForWhichFairValueOptionWasElectedAssets
Namespace Prefix:	fcnca_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

For assets for which the fair value option has been elected, this item represents the difference between the fair value and the aggregate unpaid principal balance for those assets.

+ References

No definition available.

+ Details

Name:	fcnca_FairValueOptionAggregateDifferenceAssets
Namespace Prefix:	fcnca_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

For each line item in the statement of financial position, the amounts of gains and losses from fair value changes included in earnings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=75031198&loc=d3e14172-108612

+ Details

Name:	us-gaap_FairValueOptionChangesInFairValueGainLoss1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

No definition available.

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=fcnca_LoansHeldForSaleMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: