| Document and Entity Information | 6 Months Ended | |
| Mar. 31, 2015 | Apr. 29, 2015 | |
| Document and Entity Information | ||
| Entity Registrant Name | Westbury Bancorp, Inc. | |
| Entity Central Index Key | 1553326 | |
| Document Type | 10-Q | |
| Document Period End Date | 31-Mar-15 | |
| Amendment Flag | FALSE | |
| Current Fiscal Year End Date | -21 | |
| Entity Current Reporting Status | Yes | |
| Entity Filer Category | Smaller Reporting Company | |
| Entity Common Stock, Shares Outstanding | 4,528,049 | |
| Document Fiscal Year Focus | 2015 | |
| Document Fiscal Period Focus | Q2 |
