| Document and Entity Information | Apr. 30, 2024 |
| Document and Entity Information [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Apr. 30, 2024 |
| Entity File Number | 0-16759 |
| Entity Registrant Name | FIRST FINANCIAL CORPORATION |
| Entity Incorporation, State or Country Code | IN |
| Entity Tax Identification Number | 35-1546989 |
| Entity Address State Or Province | IN |
| Entity Address, Address Line One | One First Financial Plaza |
| Entity Address, City or Town | Terre Haute |
| Entity Address, Postal Zip Code | 47807 |
| City Area Code | 812 |
| Local Phone Number | 238-6000 |
| Title of 12(b) Security | Common Stock, par value $0.125 per share |
| Trading Symbol | THFF |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000714562 |
| Amendment Flag | false |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
