The number of shares of an investment sold during the period.

No definition available.

Name:	fhb_InvestmentOwnedSharesSold
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Data Type:	xbrli:sharesItemType
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Period Type:	duration

- Definition

The parent entity's interest in net assets of the subsidiary, expressed as a percentage.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Cash received on stock transaction after deduction of issuance costs.

+ References

No definition available.

+ Details

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

- Definition

Percentage of subsidiary's or equity investee's stock owned by parent company after stock transaction.

+ References

No definition available.

+ Details

Name:	us-gaap_SaleOfStockPercentageOfOwnershipAfterTransaction
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
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

No definition available.

+ Details

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

- Details

Name:	srt_OwnershipAxis=fhb_FirstHawaiianIncorporatedMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	dei_LegalEntityAxis=fhb_BnpParibasMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsidiarySaleOfStockAxis=us-gaap_IPOMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsidiarySaleOfStockAxis=fhb_SecondaryOfferingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsidiarySaleOfStockAxis=us-gaap_RepurchaseAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsidiarySaleOfStockAxis=fhb_CompletionOfPublicOfferingMember
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Organization and Basis of Presentation - Transition to an Independent Public Company (Details) - First Hawaiian, Inc. (FHI) - USD ($)				1 Months Ended				12 Months Ended
	Feb. 01, 2019	Feb. 17, 2017	Aug. 09, 2016	Sep. 30, 2018	Aug. 31, 2018	Jun. 30, 2018	May 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reorganization Transactions
Outstanding common stock owned (as a percent)								100.00%
BNP Paribas (BNPP)
Reorganization Transactions
Outstanding common stock owned after sales of shares (as a percent)	0.00%
Initial public offering \| BNP Paribas (BNPP)
Reorganization Transactions
Number of shares sold			24,250,000
Secondary offering \| BNP Paribas (BNPP)
Reorganization Transactions
Number of shares sold		28,750,000		20,000,000	20,000,000	16,830,000	16,830,000
Share repurchase agreements \| BNP Paribas (BNPP)
Reorganization Transactions
Number of shares sold					1,801,801		2,968,069
Completion of public offering
Reorganization Transactions
Proceeds from the sales of shares								$ 0	$ 0	$ 0
Completion of public offering \| BNP Paribas (BNPP)
Reorganization Transactions
Number of shares sold	24,859,750