XML 119 R1.htm IDEA: XBRL DOCUMENT v3.19.3.a.u2
Cover page Cover page - USD ($)
 12 Months Ended
Dec. 31, 2019
Feb. 07, 2020
Jun. 28, 2019
Cover page.      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2019    
Document Transition Report false    
Entity File Number 1-10499    
Entity Registrant Name NORTHWESTERN CORP    
Entity Central Index Key 0000073088    
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY    
Amendment Flag false    
Entity Incorporation, State or Country Code DE    
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    
Title of 12(b) Security Common stock    
Trading Symbol NWE    
Security Exchange Name NYSE    
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    
Entity Shell Company false    
Entity Public Float     $ 3,639,448,000
Entity Common Stock, Shares Outstanding   50,478,630  
Documents Incorporated by Reference
Documents Incorporated by Reference
Certain sections of our Proxy Statement for the 2020 Annual Meeting of Shareholders
are incorporated by reference into Part III of this Form 10-K