| Document and Entity Information | Oct. 22, 2021 |
| Document and Entity Information [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Oct. 22, 2021 |
| Entity File Number | 001-14585 |
| Entity Registrant Name | FIRST HAWAIIAN, INC. |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 99-0156159 |
| Entity Address, Address Line One | 999 Bishop St. |
| Entity Address, Adress Line Two | 29th Floor |
| Entity Address, State or Province | HI |
| Entity Address, City or Town | Honolulu |
| Entity Address, Postal Zip Code | 96813 |
| City Area Code | 808 |
| Local Phone Number | 525-7000 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | FHB |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000036377 |
| Amendment Flag | false |
