Allowance for Loan and Lease Losses (Allocation of Allowance for Loan and Lease Losses) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	$ 1,175,441	$ 1,266,395
Allowance for loan and lease losses	221,893	218,795	$ 206,216	$ 204,466
Non-PCI impaired loans less than $500,00 collectively evaluated	49,100	47,400
Non-PCI Loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	170,034	150,189
ALLL for loans and leases individually evaluated for impairment	9,728	9,210
Impaired Financing Receivable, with Related Allowance, Recorded Investment	77,199	79,491
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	78,447	56,908
Financing Receivable, Individually Evaluated for Impairment	155,646	136,399
Average balance	141,733	135,982	136,404
Interest income recognized	4,790	4,466	4,675
Allowance for loan and lease losses	211,867	205,026	189,904	182,837
Non-PCI Loans \| Commercial \| Other commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	2,267	1,880
ALLL for loans and leases individually evaluated for impairment	209	152
Impaired Financing Receivable, with Related Allowance, Recorded Investment	1,351	1,279
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	506	284
Financing Receivable, Individually Evaluated for Impairment	1,857	1,563
Average balance	1,642	1,112	481
Interest income recognized	34	38	12
Allowance for loan and lease losses	4,571	3,269	2,168	732
Non-PCI Loans \| Commercial \| Construction and land development
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	1,110	1,172
ALLL for loans and leases individually evaluated for impairment	185	151
Impaired Financing Receivable, with Related Allowance, Recorded Investment	788	1,002
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	0	43
Financing Receivable, Individually Evaluated for Impairment	788	1,045
Average balance	858	2,700	3,164
Interest income recognized	37	138	146
Allowance for loan and lease losses	24,470	28,877	16,288	11,961
Non-PCI Loans \| Commercial \| Mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	78,936	82,658
ALLL for loans and leases individually evaluated for impairment	3,648	3,488
Impaired Financing Receivable, with Related Allowance, Recorded Investment	39,135	42,875
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	34,520	33,486
Financing Receivable, Individually Evaluated for Impairment	73,655	76,361
Average balance	73,815	82,146	89,934
Interest income recognized	2,596	2,671	3,129
Allowance for loan and lease losses	45,005	48,278	69,896	85,189
Non-PCI Loans \| Commercial \| Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	10,475	16,637
ALLL for loans and leases individually evaluated for impairment	665	1,732
Impaired Financing Receivable, with Related Allowance, Recorded Investment	6,326	8,920
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	1,648	3,680
Financing Receivable, Individually Evaluated for Impairment	7,974	12,600
Average balance	9,847	11,878	14,587
Interest income recognized	376	417	510
Allowance for loan and lease losses	53,697	50,225	43,116	30,727
Non-PCI Loans \| Commercial \| Lease financing
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	2,571	1,074
ALLL for loans and leases individually evaluated for impairment	397	75
Impaired Financing Receivable, with Related Allowance, Recorded Investment	1,890	1,002
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	24	72
Financing Receivable, Individually Evaluated for Impairment	1,914	1,074
Average balance	1,753	1,307	1,718
Interest income recognized	51	63	74
Allowance for loan and lease losses	6,127	5,907	5,524	4,286
Non-PCI Loans \| Commercial \| Other
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	521	233
ALLL for loans and leases individually evaluated for impairment	0	23
Impaired Financing Receivable, with Related Allowance, Recorded Investment	0	142
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	521	0
Financing Receivable, Individually Evaluated for Impairment	521	142
Average balance	426	687	1,673
Interest income recognized	22	33	37
Allowance for loan and lease losses	4,689	3,127	1,855	3,184
Non-PCI Loans \| Noncommercial \| Construction and land development
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	5,224	3,030
ALLL for loans and leases individually evaluated for impairment	68	109
Impaired Financing Receivable, with Related Allowance, Recorded Investment	592	645
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	3,959	1,968
Financing Receivable, Individually Evaluated for Impairment	4,551	2,613
Average balance	3,383	983	829
Interest income recognized	145	50	38
Allowance for loan and lease losses	3,962	1,596	1,485	892
Non-PCI Loans \| Noncommercial \| Mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	39,946	32,588
ALLL for loans and leases individually evaluated for impairment	2,733	2,447
Impaired Financing Receivable, with Related Allowance, Recorded Investment	19,135	20,269
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	18,707	11,207
Financing Receivable, Individually Evaluated for Impairment	37,842	31,476
Average balance	33,818	26,774	18,524
Interest income recognized	990	805	557
Allowance for loan and lease losses	15,706	14,447	14,105	10,661
Non-PCI Loans \| Noncommercial \| Revolving mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	25,941	8,831
ALLL for loans and leases individually evaluated for impairment	1,085	366
Impaired Financing Receivable, with Related Allowance, Recorded Investment	5,875	1,825
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	17,895	5,788
Financing Receivable, Individually Evaluated for Impairment	23,770	7,613
Average balance	14,022	6,915	4,368
Interest income recognized	436	171	97
Allowance for loan and lease losses	22,436	21,013	15,971	18,650
Non-PCI Loans \| Noncommercial \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	3,043	2,086
ALLL for loans and leases individually evaluated for impairment	738	667
Impaired Financing Receivable, with Related Allowance, Recorded Investment	2,107	1,532
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	667	380
Financing Receivable, Individually Evaluated for Impairment	2,774	1,912
Average balance	2,169	1,480	1,126
Interest income recognized	103	80	75
Allowance for loan and lease losses	31,204	28,287	19,496	16,555
PCI Loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Acquired loans which have adverse change in expected cash flows	279,812	359,699
Allowance for loan and lease losses	10,026	13,769	16,312	21,629
PCI Loans \| Commercial \| Other commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan and lease losses	221	502	773	75
PCI Loans \| Commercial \| Construction and land development
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan and lease losses	335	483	1,082	150
PCI Loans \| Commercial \| Mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan and lease losses	6,564	6,423	7,838	10,135
PCI Loans \| Commercial \| Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan and lease losses	306	504	445	1,240
PCI Loans \| Noncommercial \| Construction and land development
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan and lease losses	0	0	0	183
PCI Loans \| Noncommercial \| Mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan and lease losses	2,117	4,818	5,398	5,820
PCI Loans \| Noncommercial \| Revolving mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Allowance for loan and lease losses	$ 259	$ 956	$ 523	$ 3,999

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://www.xbrl.org/2003/role/presentationRef
-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://www.xbrl.org/2003/role/presentationRef
-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://www.xbrl.org/2003/role/presentationRef
-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://www.xbrl.org/2003/role/presentationRef
-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://www.xbrl.org/2003/role/presentationRef
-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://www.xbrl.org/2003/role/presentationRef
-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=us-gaap_FinanceLeasesPortfolioSegmentMember
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_ReceivablesAcquiredWithDeterioratedCreditQualityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: