Estimated Fair Values (Fair Value Option) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value, Option, Changes in Fair Value, Gain (Loss)	$ 50	$ 2,900	$ (2,400)
Loans Held For Sale
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair Value Of Items For Which Fair Value Option Was Elected Assets	45,505	51,179
Aggregate Unpaid Principal Balance Of Items For Which Fair Value Option Was Elected Assets	44,073	49,796
Fair Value Option Aggregate Difference Assets	$ 1,432	$ 1,383

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

No definition available.

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- Definition

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

+ References

No definition available.

+ Details

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- 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

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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

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Balance Type:	credit
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- 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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- Details

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