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v3.25.4

SEGMENT INFORMATION - Narrative (Details) | 12 Months Ended
--- | ---
Dec. 31, 2025
Segment Reporting, Asset Reconciling Item [Line Items] |
Number Of Reportable Segments, Not Disclosed Flag | true
Operating Lease Equipment | Minimum |
Segment Reporting, Asset Reconciling Item [Line Items] |
Useful Life (years) | 3 years
Operating Lease Equipment | Maximum |
Segment Reporting, Asset Reconciling Item [Line Items] |
Useful Life (years) | 10 years
Rail Equipment | Minimum |
Segment Reporting, Asset Reconciling Item [Line Items] |
Useful Life (years) | 40 years
Rail Equipment | Maximum |
Segment Reporting, Asset Reconciling Item [Line Items] |
Useful Life (years) | 50 years

X
- Definition Number Of Reportable Segments, Not Disclosed Flag + References No definition available. + Details
Name: | fcnca_NumberOfReportableSegmentsNotDisclosedFlag
Namespace Prefix: | fcnca_
Data Type: | xbrli:booleanItemType
Balance Type: | na
Period Type: | duration

X
- Definition Useful life of long lived, physical assets used in the normal conduct of business and not intended for resale, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. Examples include, but not limited to, land, buildings, machinery and equipment, office equipment, furniture and fixtures, and computer equipment. + References No definition available. + Details
Name: | us-gaap_PropertyPlantAndEquipmentUsefulLife
Namespace Prefix: | us-gaap_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | instant

X
- 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
Name: | us-gaap_SegmentReportingAssetReconcilingItemLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=fcnca_OperatingLeaseEquipmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_PropertyPlantAndEquipmentByTypeAxis=fcnca_RailEquipmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |