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Allowance for Credit Losses - Schedule of Allowance for Credit Losses on Unfunded Commitments (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2023 | Mar. 31, 2022
Financing Receivable, Allowance for Credit Loss [Roll Forward] |  |
Allowance for credit losses, beginning balance | $ 922 | $ 178
Day 2 provision for loans and leases | 462 | 454
(Benefit) for credit losses - loans and leases |  | (53)
Provision for credit losses - loans and leases | 71 |
Total provision for credit losses - unfunded commitments | 783 | 464
Allowance for credit losses, ending balance | 1,605 | 848
Unfunded Commitments |  |
Financing Receivable, Allowance for Credit Loss [Roll Forward] |  |
Allowance for credit losses, beginning balance | 106 | 12
Day 2 provision for loans and leases | 254 | 59
(Benefit) for credit losses - loans and leases | (8) |
Provision for credit losses - loans and leases |  | 4
Total provision for credit losses - unfunded commitments | 246 | 63
Allowance for credit losses, ending balance | $ 352 | $ 75

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- Definition Financing Receivable, Credit Loss, Expense + References No definition available. + Details
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Namespace Prefix: | fcnca_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Financing Receivable, Credit Loss, Reversal + References No definition available. + Details
Name: | fcnca_FinancingReceivableCreditLossReversal
Namespace Prefix: | fcnca_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Financing Receivable, Excluding Accrued Interest, Purchased without Credit Deterioration, Allowance for Credit Loss at Acquisition Date + References No definition available. + Details
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Namespace Prefix: | fcnca_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- 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 -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 45 -Paragraph 1 -URI https://asc.fasb.org/extlink&oid=124255206&loc=SL82895884-210446 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- 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_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount, excluding accrued interest, of credit loss expense (reversal of expense) on financing receivable. Excludes net investment in lease. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 326 -SubTopic 20 -Section 50 -Paragraph 13 -Subparagraph (b) -URI https://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
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Data Type: | na
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