| Document and Entity Information Document | Jun. 29, 2023 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Jun. 29, 2023 |
| Entity Registrant Name | NorthWestern Corporation |
| Entity Central Index Key | 0000073088 |
| Amendment Flag | false |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 1-10499 |
| Entity Tax Identification Number | 46-0172280 |
| 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common stock |
| Trading Symbol | NWE |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
