Document and Entity Information Document - shares	3 Months Ended	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2021	Oct. 22, 2021
Cover [Abstract]
Document Type		10-Q
Document Quarterly Report		true
Document Period End Date	Sep. 30, 2021
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		2021
Document Fiscal Period Focus		Q3
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		NASDAQ
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 Common Stock, Shares Outstanding			52,653,671