Variable Interest Entities - Narrative (Details) - entity	Dec. 31, 2023	Dec. 31, 2022
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Variable interest entity, consolidated, number of entities	0	0

Variable Interest Entity, Consolidated, Number Of Entities

No definition available.

Name:	fcnca_VariableInterestEntityConsolidatedNumberOfEntities
Namespace Prefix:	fcnca_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant