| Cover | Aug. 28, 2023 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Aug. 28, 2023 |
| Entity File Number | 001-34535 |
| Entity Registrant Name | UNITED STATES 12 MONTH NATURAL GAS FUND, LP |
| Entity Central Index Key | 0001405513 |
| Entity Tax Identification Number | 26-0431733 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | 1850 Mt. Diablo Boulevard |
| Entity Address, Address Line Two | Suite 640 |
| Entity Address, City or Town | Walnut Creek |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 94596 |
| City Area Code | 510 |
| Local Phone Number | 522-9600 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Shares of United States 12 Month Natural Gas Fund, LP |
| Trading Symbol | UNL |
| Security Exchange Name | NYSEArca |
| Entity Emerging Growth Company | false |
