Allowance for Loan and Lease Losses (Allocation of Allowance for Loan and Lease Losses) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	$ 960,457	$ 1,175,441	$ 1,266,395
Non-PCI impaired loans less than $500,00 collectively evaluated	47,100	49,100
Non-PCI Loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	163,902	170,034
ALLL for loans and leases individually evaluated for impairment	9,827	9,728
Impaired Financing Receivable, with Related Allowance, Recorded Investment	115,073	77,199
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	31,357	78,447
Financing Receivable, Individually Evaluated for Impairment	146,430	155,646
Average balance	153,123	141,733	135,982
Interest income recognized	5,488	4,790	4,466
Allowance for loan and lease losses	214,568	211,867	205,026	$ 189,904
Non-PCI Loans \| Commercial \| Other commercial real estate
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	946	2,267
ALLL for loans and leases individually evaluated for impairment	42	209
Impaired Financing Receivable, with Related Allowance, Recorded Investment	243	1,351
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	617	506
Financing Receivable, Individually Evaluated for Impairment	860	1,857
Average balance	1,225	1,642	1,112
Interest income recognized	43	34	38
Allowance for loan and lease losses	2,481	4,571	3,269	2,168
Non-PCI Loans \| Commercial \| Construction and land development
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	2,606	1,110
ALLL for loans and leases individually evaluated for impairment	490	185
Impaired Financing Receivable, with Related Allowance, Recorded Investment	1,897	788
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	278	0
Financing Receivable, Individually Evaluated for Impairment	2,175	788
Average balance	1,734	858	2,700
Interest income recognized	84	37	138
Allowance for loan and lease losses	35,270	24,470	28,877	16,288
Non-PCI Loans \| Commercial \| Mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	61,317	78,936
ALLL for loans and leases individually evaluated for impairment	2,671	3,648
Impaired Financing Receivable, with Related Allowance, Recorded Investment	34,177	39,135
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	21,270	34,520
Financing Receivable, Individually Evaluated for Impairment	55,447	73,655
Average balance	65,943	73,815	82,146
Interest income recognized	2,569	2,596	2,671
Allowance for loan and lease losses	43,451	45,005	48,278	69,896
Non-PCI Loans \| Commercial \| Commercial and industrial
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	14,695	13,046
ALLL for loans and leases individually evaluated for impairment	1,137	1,062
Impaired Financing Receivable, with Related Allowance, Recorded Investment	7,153	8,216
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	2,715	1,672
Financing Receivable, Individually Evaluated for Impairment	9,868	9,888
Average balance	9,560	11,600	13,185
Interest income recognized	364	427	480
Allowance for loan and lease losses	55,620	59,824	56,132	48,640
Non-PCI Loans \| Commercial \| Other
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	301	521
ALLL for loans and leases individually evaluated for impairment	105	0
Impaired Financing Receivable, with Related Allowance, Recorded Investment	216	0
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	75	521
Financing Receivable, Individually Evaluated for Impairment	291	521
Average balance	135	426	687
Interest income recognized	3	22	33
Allowance for loan and lease losses	2,221	4,689	3,127	1,855
Non-PCI Loans \| Noncommercial \| Construction and land development
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	4,035	5,224
ALLL for loans and leases individually evaluated for impairment	81	68
Impaired Financing Receivable, with Related Allowance, Recorded Investment	2,337	592
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	1,412	3,959
Financing Receivable, Individually Evaluated for Impairment	3,749	4,551
Average balance	3,677	3,383	983
Interest income recognized	172	145	50
Allowance for loan and lease losses	2,350	3,962	1,596	1,485
Non-PCI Loans \| Noncommercial \| Mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	45,226	39,946
ALLL for loans and leases individually evaluated for impairment	1,901	2,733
Impaired Financing Receivable, with Related Allowance, Recorded Investment	40,359	19,135
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	1,809	18,707
Financing Receivable, Individually Evaluated for Impairment	42,168	37,842
Average balance	41,368	33,818	26,774
Interest income recognized	1,237	990	805
Allowance for loan and lease losses	15,472	15,706	14,447	14,105
Non-PCI Loans \| Noncommercial \| Revolving mortgage
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	31,371	25,941
ALLL for loans and leases individually evaluated for impairment	2,515	1,085
Impaired Financing Receivable, with Related Allowance, Recorded Investment	25,751	5,875
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	3,101	17,895
Financing Receivable, Individually Evaluated for Impairment	28,852	23,770
Average balance	26,759	14,022	6,915
Interest income recognized	900	436	171
Allowance for loan and lease losses	21,862	22,436	21,013	15,971
Non-PCI Loans \| Noncommercial \| Consumer
Financing Receivable, Allowance for Credit Losses [Line Items]
Unpaid principal balance	3,405	3,043
ALLL for loans and leases individually evaluated for impairment	885	738
Impaired Financing Receivable, with Related Allowance, Recorded Investment	2,940	2,107
Impaired Financing Receivable, with No Related Allowance, Recorded Investment	80	667
Financing Receivable, Individually Evaluated for Impairment	3,020	2,774
Average balance	2,722	2,169	1,480
Interest income recognized	116	103	80
Allowance for loan and lease losses	35,841	31,204	28,287	19,496
PCI Loans
Financing Receivable, Allowance for Credit Losses [Line Items]
Acquired loans which have adverse change in expected cash flows	186,648	279,812
Allowance for loan and lease losses	$ 9,144	$ 10,026	$ 13,769	$ 16,312

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: