Estimated Fair Values (Fair Value Option) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Fair Value, Option, Quantitative Disclosures [Line Items]
Gains (Losses) from fair value changes on originated loans held for sale	$ (773)	$ 104	$ (528)	$ 3,556
Loans Held For Sale
Fair Value, Option, Quantitative Disclosures [Line Items]
Fair value of originated loans held for sale	46,082		46,082		$ 51,179
Aggregate unpaid principle balance of originated loans held for sale	45,221		45,221		49,796
Difference between fair value and aggregate unpaid principle balance of originated loans held for sale	$ 861		$ 861		$ 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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Data Type:	xbrli:monetaryItemType
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Period Type:	instant

- 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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- 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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