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Cover page Cover page - USD ($)
 12 Months Ended
Dec. 31, 2024
Feb. 07, 2025
Jun. 30, 2024
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2024    
Document Transition Report false    
Entity File Number 000-56598    
Entity Registrant Name NORTHWESTERN ENERGY GROUP, INC.    
Entity Central Index Key 0001993004    
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2024    
Document Fiscal Period Focus FY    
Amendment Flag false    
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    
Title of 12(b) Security Common stock    
Trading Symbol NWE    
Security Exchange Name NASDAQ    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Entity Shell Company false    
Entity Public Float     $ 3,069,877,808
Entity Common Stock, Shares Outstanding   61,328,041  
Documents Incorporated by Reference
Documents Incorporated by Reference
Certain sections of our Proxy Statement for the 2025 Annual Meeting of Shareholders are incorporated by reference into Part III of this Form 10-K
    
Document Financial Statement Error Correction [Flag] false