Mortgage Servicing Rights (Narrative) (Details) - USD ($)	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Mortgage Servicing Rights [Abstract]
Residential Mortgage Loans Serviced	$ 3,410,000,000		$ 3,380,000,000
Contractually Specified Servicing Fee, Late Fee, and Ancillary Fee Earned in Exchange for Servicing Financial Asset	2,100,000	$ 1,900,000
Other servicing rights	1,700,000		$ 1,900,000
Valuation allowance increase	$ 2,967,000	$ 161,000

Mortgage Servicing Rights [Abstract]

No definition available.

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- Definition

Unpaid principal balance of residential mortgage loans serviced for others at period end.

+ References

No definition available.

+ Details

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

- Definition

Servicing Asset, Other

+ References

No definition available.

+ Details

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

- Definition

Amount of contractually specified servicing fee, late fee, and ancillary fee recognized as income for servicing asset and servicing liability.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=116651436&loc=d3e122596-111746

+ Details

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Balance Type:	credit
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- Definition

Amount of increase (decrease) in the valuation allowance for impairment of contract to service financial assets under which the benefits of servicing are expected to more than adequately compensate the servicer from expenses (recoveries) recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 4
-Subparagraph (e)(2)
-URI http://asc.fasb.org/extlink&oid=116651436&loc=d3e122739-111746

+ Details

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