Document and Entity Information Document - shares	6 Months Ended
	Jun. 30, 2019	Jul. 19, 2019
Cover page.
Document Type	10-Q
Document Transition Report	false
Document Period End Date	Jun. 30, 2019
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	Q2
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 Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Shell Company	false
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	true
Entity Common Stock, Shares Outstanding		50,442,932