ALLOWANCE FOR LOAN AND LEASE LOSSES - Schedule of Allowance for Loan and Lease Losses (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2026	Mar. 31, 2025
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan and lease losses, beginning balance	$ 1,566	$ 1,676
Provision for loan and lease losses	103	148
Charge-offs	(132)	(167)
Recoveries	21	23
Allowance for loan and lease losses, ending balance	1,558	1,680
Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan and lease losses, beginning balance	1,436	1,518
Provision for loan and lease losses	97	138
Charge-offs	(123)	(159)
Recoveries	18	20
Allowance for loan and lease losses, ending balance	1,428	1,517
Consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan and lease losses, beginning balance	130	158
Provision for loan and lease losses	6	10
Charge-offs	(9)	(8)
Recoveries	3	3
Allowance for loan and lease losses, ending balance	$ 130	$ 163

Financing Receivable, Credit Loss, Expense

No definition available.

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

- Definition

Amount excluding accrued interest, of allowance for credit loss on financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479344/326-20-45-1

+ Details

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

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	na
Period Type:	duration

- Definition

Amount, excluding accrued interest, of increase in allowance for credit loss on financing receivable from recovery. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

+ Details

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

- Definition

Amount, excluding accrued interest, of decrease in allowance for credit loss on financing receivable from writeoff. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-6

+ Details

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Balance Type:	debit
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- Details

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- Details

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