| Cover | Mar. 16, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Mar. 16, 2021 |
| Entity File Number | 001-34155 |
| Entity Registrant Name | FIRST SAVINGS FINANCIAL GROUP, INC. |
| Entity Central Index Key | 0001435508 |
| Entity Tax Identification Number | 37-1567871 |
| Entity Incorporation, State or Country Code | IN |
| Entity Address, Address Line One | 702 North Shore Drive |
| Entity Address, Address Line Two | Suite 300 |
| Entity Address, City or Town | Jeffersonville |
| Entity Address, State or Province | IN |
| Entity Address, Postal Zip Code | 47130 |
| City Area Code | 812 |
| Local Phone Number | 283-0724 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.01 per share |
| Trading Symbol | FSFG |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
