| Document and Entity Information Document | Apr. 26, 2024 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Entity Registrant Name | NorthWestern Energy Group, Inc. |
| Entity File Number | 000-56598 |
| Entity Incorporation, State or Country Code | DE |
| Entity Tax Identification Number | 93-2020320 |
| Entity Address, Address Line One | 3010 W. 69th Street |
| Entity Address, City or Town | Sioux Falls |
| Entity Address, State or Province | SD |
| Entity Address, Postal Zip Code | 57108 |
| City Area Code | 605 |
| Local Phone Number | 978-2900 |
| Entity Emerging Growth Company | false |
| Title of 12(b) Security | Common stock |
| Entity Central Index Key | 0001993004 |
| Amendment Flag | false |
| Trading Symbol | NWE |
| Security Exchange Name | NASDAQ |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Document Period End Date | Apr. 26, 2024 |
