| Document and Entity Information | Sep. 30, 2021 |
| Document And Entity Information [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Sep. 30, 2021 |
| Entity Registrant Name | Anterix Inc. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-36827 |
| Entity Tax Identification Number | 33-0745043 |
| Entity Address, Address Line One | 3 Garret Mountain Plaza |
| Entity Address, Address Line Two | Suite 401 |
| Entity Address, City or Town | Woodland Park |
| Entity Address, State or Province | NJ |
| Entity Address, Postal Zip Code | 07424 |
| City Area Code | 973 |
| Local Phone Number | 771-0300 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, $0.0001 par value |
| Trading Symbol | ATEX |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Amendment Flag | false |
| Entity Central Index Key | 0001304492 |
