Allowance for Loan and Lease Losses (Allocation of Allowance for Loan and Lease Losses) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	$ 768,391	$ 960,457	$ 1,175,441
Non-PCI impaired loans less than $500,00 collectively evaluated	41,000	47,100
Non-PCI Loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	200,599	163,902
ALLL for loans and leases individually evaluated for impairment	12,915	9,827
Impaired Financing Receivable, with Related Allowance, Recorded Investment	133,358	115,073
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	52,241	31,357
Financing Receivable, Individually Evaluated for Impairment	185,599	146,430
Average balance	169,938	153,123	141,733
Interest income recognized	5,410	5,488	4,790
Allowance for loan and lease losses	217,605	214,568	211,867	$ 205,026
Non-PCI Loans \| Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	95,927	79,865
ALLL for loans and leases individually evaluated for impairment	5,634	4,445
Impaired Financing Receivable, with Related Allowance, Recorded Investment	52,516	43,686
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	36,377	24,955
Financing Receivable, Individually Evaluated for Impairment	88,893	68,641
Average balance	81,477	78,597	88,341
Interest income recognized	2,761	3,063	3,116
Non-PCI Loans \| Commercial \| Other commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	1,360	946
ALLL for loans and leases individually evaluated for impairment	39	42
Impaired Financing Receivable, with Related Allowance, Recorded Investment	318	243
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	950	617
Financing Receivable, Individually Evaluated for Impairment	1,268	860
Average balance	919	1,225	1,642
Interest income recognized	27	43	34
Allowance for loan and lease losses	2,211	2,481	4,571	3,269
Non-PCI Loans \| Commercial \| Construction and land development
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	5,109	2,606
ALLL for loans and leases individually evaluated for impairment	463	490
Impaired Financing Receivable, with Related Allowance, Recorded Investment	1,851	1,897
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	2,804	278
Financing Receivable, Individually Evaluated for Impairment	4,655	2,175
Average balance	3,915	1,734	858
Interest income recognized	53	84	37
Allowance for loan and lease losses	33,213	35,270	24,470	28,877
Non-PCI Loans \| Commercial \| Mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	74,804	61,317
ALLL for loans and leases individually evaluated for impairment	3,650	2,671
Impaired Financing Receivable, with Related Allowance, Recorded Investment	42,394	34,177
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	27,755	21,270
Financing Receivable, Individually Evaluated for Impairment	70,149	55,447
Average balance	64,363	65,943	73,815
Interest income recognized	2,188	2,569	2,596
Allowance for loan and lease losses	45,335	43,451	45,005	48,278
Non-PCI Loans \| Commercial \| Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	13,993	14,695
ALLL for loans and leases individually evaluated for impairment	1,379	1,137
Impaired Financing Receivable, with Related Allowance, Recorded Investment	7,547	7,153
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	4,635	2,715
Financing Receivable, Individually Evaluated for Impairment	12,182	9,868
Average balance	11,884	9,560	11,600
Interest income recognized	482	364	427
Allowance for loan and lease losses	59,374	55,620	59,824	56,132
Non-PCI Loans \| Commercial \| Other
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	661	301
ALLL for loans and leases individually evaluated for impairment	103	105
Impaired Financing Receivable, with Related Allowance, Recorded Investment	406	216
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	233	75
Financing Receivable, Individually Evaluated for Impairment	639	291
Average balance	396	135	426
Interest income recognized	11	3	22
Allowance for loan and lease losses	2,236	2,221	4,689	3,127
Non-PCI Loans \| Noncommercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	104,672	84,037
ALLL for loans and leases individually evaluated for impairment	7,281	5,382
Impaired Financing Receivable, with Related Allowance, Recorded Investment	80,842	71,387
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	15,864	6,402
Financing Receivable, Individually Evaluated for Impairment	96,706	77,789
Average balance	88,461	74,526	53,392
Interest income recognized	2,649	2,425	1,674
Non-PCI Loans \| Noncommercial \| Construction and land development
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	4,150	4,035
ALLL for loans and leases individually evaluated for impairment	174	81
Impaired Financing Receivable, with Related Allowance, Recorded Investment	2,470	2,337
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	1,412	1,412
Financing Receivable, Individually Evaluated for Impairment	3,882	3,749
Average balance	3,589	3,677	3,383
Interest income recognized	116	172	145
Allowance for loan and lease losses	2,709	2,350	3,962	1,596
Non-PCI Loans \| Noncommercial \| Mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	64,741	45,226
ALLL for loans and leases individually evaluated for impairment	3,278	1,901
Impaired Financing Receivable, with Related Allowance, Recorded Investment	48,796	40,359
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	11,646	1,809
Financing Receivable, Individually Evaluated for Impairment	60,442	42,168
Average balance	52,045	41,368	33,818
Interest income recognized	1,386	1,237	990
Allowance for loan and lease losses	18,232	15,472	15,706	14,447
Non-PCI Loans \| Noncommercial \| Revolving mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	31,960	31,371
ALLL for loans and leases individually evaluated for impairment	2,722	2,515
Impaired Financing Receivable, with Related Allowance, Recorded Investment	26,104	25,751
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	2,765	3,101
Financing Receivable, Individually Evaluated for Impairment	28,869	28,852
Average balance	29,516	26,759	14,022
Interest income recognized	1,009	900	436
Allowance for loan and lease losses	19,702	21,862	22,436	21,013
Non-PCI Loans \| Noncommercial \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	3,821	3,405
ALLL for loans and leases individually evaluated for impairment	1,107	885
Impaired Financing Receivable, with Related Allowance, Recorded Investment	3,472	2,940
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	41	80
Financing Receivable, Individually Evaluated for Impairment	3,513	3,020
Average balance	3,311	2,722	2,169
Interest income recognized	138	116	103
Allowance for loan and lease losses	34,593	35,841	31,204	28,287
PCI Loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Acquired loans which have adverse change in expected cash flows	139,400	186,600
Allowance for loan and lease losses	$ 7,536	$ 9,144	$ 10,026	$ 13,769

Acquired Loans Which Have Adverse Change In Expected Cash Flows

No definition available.

Name:	fcnca_AcquiredLoansWhichHaveAdverseChangeInExpectedCashFlows
Namespace Prefix:	fcnca_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The recorded investment of impaired financing receivables less than $500,000 that are collectively evaluated.

+ References

No definition available.

+ Details

Name:	fcnca_ImpairedFinancingReceivableCollectivelyEvaluated
Namespace Prefix:	fcnca_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

A valuation allowance for financing receivables that are expected to be uncollectible.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLosses
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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_FinancingReceivableAllowanceForCreditLossesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

+ Details

Name:	us-gaap_FinancingReceivableIndividuallyEvaluatedForImpairment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5212-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5258-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableAverageRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableInterestIncomeAccrualMethod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of unpaid principal balance of impaired financing receivables with related allowance for credit losses and without related allowance for credit losses.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableUnpaidPrincipalBalance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in impaired financing receivables for which there is no related allowance for credit losses.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953550-111524

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithNoRelatedAllowanceRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

Name:	us-gaap_ImpairedFinancingReceivableWithRelatedAllowanceRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	fcnca_LoanDesignationAxis=fcnca_NonPCILoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=fcnca_OtherRealEstateLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=us-gaap_MortgageReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=fcnca_CommercialAndIndustrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=fcnca_OtherLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fcnca_NoncommercialSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=us-gaap_HomeEquityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis=us-gaap_ConsumerLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	fcnca_LoanDesignationAxis=us-gaap_FinancialAssetAcquiredWithCreditDeteriorationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: