ALLOWANCE FOR LOAN AND LEASE LOSSES - Schedule of Allowance for Loan and Lease Losses (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2026	Jun. 30, 2025
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan and lease losses, beginning balance	$ 1,558	$ 1,680	$ 1,566	$ 1,676
Provision for loan and lease losses	34	111	137	259
Charge-offs	(130)	(144)	(262)	(311)
Recoveries	22	25	43	48
Allowance for loan and lease losses, ending balance	1,484	1,672	1,484	1,672
Commercial
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan and lease losses, beginning balance	1,428	1,517	1,436	1,518
Provision for loan and lease losses	28	111	125	249
Charge-offs	(122)	(137)	(245)	(296)
Recoveries	19	21	37	41
Allowance for loan and lease losses, ending balance	1,353	1,512	1,353	1,512
Consumer
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Allowance for loan and lease losses, beginning balance	130	163	130	158
Provision for loan and lease losses	6	0	12	10
Charge-offs	(8)	(7)	(17)	(15)
Recoveries	3	4	6	7
Allowance for loan and lease losses, ending balance	$ 131	$ 160	$ 131	$ 160

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/2003/role/disclosureRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477657/942-210-S45-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478478/942-210-S50-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(7)(2))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

Reference 4: 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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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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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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- Details

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