FDIC Shared-Loss Receivable FDIC Shared-Loss Payable (Changes in Payable to FDIC) (Details) $ in Thousands	3 Months Ended
Jun. 30, 2019 USD ($)
FDIC Shared-Payable [Abstract]
Accretion expense	$ 3,274
FDIC shared-loss payable	$ 108,892

Amount of accretion expense for FDIC shared-loss payable.

No definition available.

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