| Document and Entity Information | Oct. 28, 2020 |
| Cover [Abstract] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001382230 |
| Document Type | 8-K |
| Document Period End Date | Oct. 28, 2020 |
| Entity Registrant Name | ESSA Bancorp, Inc. |
| Entity Incorporation State Country Code | PA |
| Entity File Number | 001-33384 |
| Entity Tax Identification Number | 20-8023072 |
| Entity Address, Address Line One | 200 Palmer Street |
| Entity Address, City or Town | Stroudsburg |
| Entity Address, State or Province | PA |
| Entity Address, Postal Zip Code | 18360 |
| City Area Code | (570) |
| Local Phone Number | 421-0531 |
| Security 12b Title | Common |
| Trading Symbol | ESSA |
| Security Exchange Name | NASDAQ |
| Written Communications | false |
| Soliciting Material | false |
| Pre Commencement Tender Offer | false |
| Pre Commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
