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FDIC Shared-Loss Receivable FDIC Shared-Loss Payable (Changes in Payable to FDIC) (Details) - USD ($) $ in Thousands | 3 Months Ended | 6 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Mar. 31, 2018 | Dec. 31, 2017 | Mar. 31, 2017 | Dec. 31, 2016
FDIC Shared-Payable [Abstract] |  |  |  |  |  |  |  |
FDIC shared-loss payable | $ 103,487 | $ 99,126 | $ 103,487 | $ 99,126 | $ 102,466 | $ 101,342 | $ 98,013 | $ 97,008
Accretion expense | 1,001 | 963 | 1,993 | 1,917 |  |  |  |
Adjustments related to changes in assumptions | $ 20 | $ 150 | $ 152 | $ 201 |  |  |  |

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- Definition Amount of accretion expense for FDIC shared-loss payable. + References No definition available. + Details
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Balance Type: | debit
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- Definition FDIC shared-loss payable adjustments related to changes in assumptions. + References No definition available. + Details
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Namespace Prefix: | fcnca_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition FDIC Shared-Payable [Abstract] + References No definition available. + Details
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Namespace Prefix: | fcnca_
Data Type: | xbrli:stringItemType
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Period Type: | duration

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- Definition Payable to FDIC for Loss Share Agreements + References No definition available. + Details
Name: | fcnca_PayableToFdicForLossShareAgreements
Namespace Prefix: | fcnca_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant