Transfers of Financial Assets - Carrying Amounts of Assets Pledged as Collateral, Borrowings and Other Transactions (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2024	Dec. 31, 2023
Financial Instruments Owned and Pledged as Collateral [Line Items]
In-substance debt defeasance	$ 0	$ 0
Carrying amounts of the assets pledged as collateral
Securities for reverse repurchase agreements	0	0
Asset Pledged as Collateral
Financial Instruments Owned and Pledged as Collateral [Line Items]
Assets pledged as collateral	11,027,608	11,278,394
Public deposits \| Asset Pledged as Collateral with Right
Financial Instruments Owned and Pledged as Collateral [Line Items]
Assets pledged as collateral	1,946,863	2,571,359
Federal Home Loan Bank Advances \| Asset Pledged as Collateral
Financial Instruments Owned and Pledged as Collateral [Line Items]
Assets pledged as collateral	5,000,994	4,495,266
Federal Reserve Bank Advances \| Asset Pledged as Collateral
Financial Instruments Owned and Pledged as Collateral [Line Items]
Assets pledged as collateral	3,943,124	4,074,093
ACH transactions \| Asset Pledged as Collateral without Right
Financial Instruments Owned and Pledged as Collateral [Line Items]
Assets pledged as collateral	135,443	137,101
Interest rate swaps \| Asset Pledged as Collateral without Right
Financial Instruments Owned and Pledged as Collateral [Line Items]
Assets pledged as collateral	$ 1,184	$ 575

Amount of debt extinguished by in-substance defeasance.

No definition available.

Name:	fhb_InSubstanceDebtDefeasanceAmount
Namespace Prefix:	fhb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- References

No definition available.

+ Details

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

- 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 860
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481444/860-30-45-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-04(11))
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479617/946-210-S99-1

+ Details

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

- Definition

The aggregate fair value as of the balance sheet date of financial instruments and other positions owned by the entity including: (1) mortgages, mortgage-backed and asset backed securities; (2) US government and agency obligations; (3) state and municipal government obligations; (4) other sovereign government debt; (5) corporate obligations; (6) corporate equities; (7) principal investments; (8) derivative contracts; and (9) physical commodities. Includes both pledged and unpledged holdings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 940
-SubTopic 320
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481961/940-320-45-2

+ Details

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

- Definition

The carrying value of securities purchased to be resold in reverse repurchase transactions as of the balance sheet date.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_PledgedStatusAxis=us-gaap_AssetPledgedAsCollateralMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_DepositsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PledgedStatusAxis=us-gaap_AssetPledgedAsCollateralWithRightMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_FederalHomeLoanBankAdvancesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_FederalReserveBankAdvancesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=fhb_AutomatedClearingHouseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PledgedStatusAxis=us-gaap_AssetPledgedAsCollateralWithoutRightMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: