Amount excluding accrued interest, of allowance for credit loss on financing receivable. Excludes net investment in lease.

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest
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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 13
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-13

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-K (SK)
-Number 229
-Section 1405
-Paragraph c
-Publisher SEC

+ Details

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

- Definition

Amortized cost excluding accrued interest, after allowance for credit loss, of financing receivable. Excludes net investment in lease.

+ References

+ Details

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

- Definition

Amortized cost excluding accrued interest, before allowance for credit loss, of financing receivable. Excludes net investment in lease.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479344/326-20-45-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-3B

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479319/326-20-50-5

+ Details

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

- Details

Name:	srt_CumulativeEffectPeriodOfAdoptionAxis=srt_CumulativeEffectPeriodOfAdoptionAdjustedBalanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_FinanceLeasesPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_UnallocatedFinancingReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Loans - Schedule of Loans Held for Investment (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Mar. 31, 2025	Dec. 31, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	$ 6,507,066		$ 6,376,357
Allowance for credit losses	(82,993)	$ (91,790)	(88,221)	$ (78,960)	$ (79,829)	$ (80,398)
Loans, net of allowance for credit losses	6,424,073		6,288,136
Adjusted beginning balance
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans			6,376,357
Allowance for credit losses			(88,221)
Loans, net of allowance for credit losses			6,288,136
Commercial and industrial
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	2,651,646		2,497,772
Allowance for credit losses	(36,680)	(42,671)	(37,912)	(25,398)	(28,270)	(29,523)
Commercial and industrial \| Adjusted beginning balance
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans			2,497,772
Commercial real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	1,886,358		1,912,666
Allowance for credit losses	(25,481)	(26,684)	(28,323)	(29,919)	(29,333)	(27,546)
Commercial real estate \| Adjusted beginning balance
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans			1,912,666
Commercial real estate \| Non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	705,749		752,861
Commercial real estate \| Non-owner occupied \| Adjusted beginning balance
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans			752,861
Commercial real estate \| Owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	662,120		702,773
Commercial real estate \| Owner occupied \| Adjusted beginning balance
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans			702,773
Commercial real estate \| Construction and land
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	383,969		362,677
Commercial real estate \| Construction and land \| Adjusted beginning balance
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans			362,677
Commercial real estate \| Multifamily
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	134,520		94,355
Commercial real estate \| Multifamily \| Adjusted beginning balance
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans			94,355
Residential real estate
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	1,226,760		1,180,610
Allowance for credit losses	(15,249)	(15,211)	(15,450)	(15,982)	(14,989)	(16,345)
Residential real estate \| Adjusted beginning balance
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans			1,180,610
Public finance
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	524,441		554,784
Allowance for credit losses	(3,203)	(5,243)	(4,750)	(5,921)	(5,778)	(5,337)
Public finance \| Adjusted beginning balance
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans			554,784
Consumer
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	43,080		41,345
Allowance for credit losses	(804)	(718)	(750)	(666)	(624)	(717)
Consumer \| Adjusted beginning balance
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans			41,345
Other
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	174,781		189,180
Allowance for credit losses	$ (1,576)	$ (1,263)	(1,036)	$ (1,074)	$ (835)	$ (930)
Other \| Adjusted beginning balance
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans			$ 189,180