| Cover | Oct. 27, 2020 | Jul. 29, 2020 |
| Cover [Abstract] | ||
| Document Type | 8-K | |
| Document Period End Date | Oct. 27, 2020 | |
| 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, Address Line One | One First Financial Plaza | |
| Entity Address, City or Town | Terre Haute | |
| Entity Address, State or Province | IN | |
| Entity Address, Postal Zip Code | 47807 | |
| City Area Code | (812) | |
| Local Phone Number | 238-6000 | |
| Written Communications | false | |
| Soliciting Material | false | |
| Pre-commencement Tender Offer | false | |
| Pre-commencement Issuer Tender Offer | false | |
| Entity Emerging Growth Company | false | |
| Entity Central Index Key | 0000714562 | |
| Document Fiscal Year Focus | 2020 | |
| Document Fiscal Period Focus | Q3 | |
| Amendment Flag | false | |
| Trading Symbol | THFF | |
| Title of 12(b) Security | Common Stock, par value $0.125 per share | |
| Security Exchange Name | NASDAQ |
