FDIC Shared-Loss Payable (Narrative) (Details) $ in Thousands	3 Months Ended
Mar. 31, 2021 USD ($)
FDIC Shared-Loss Receivable [Abstract]
Payment made to the FDIC to settle shared-loss agreement	$ 16,103

FDIC shared-loss payable adjustments related to changes in assumptions.

No definition available.

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