Regulatory Matters (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2020	Dec. 31, 2019	Jun. 30, 2020	Sep. 30, 2020	Dec. 31, 2019
Public Utilities, General Disclosures [Line Items]
Payment for pension benefits				$ 11.4
NorthWestern Energy Pension Plan (MT) [Member]
Public Utilities, General Disclosures [Line Items]
Payment for pension benefits				10.2
Power cost and Credit Adjustment Mechanism (PCCAM) [Member]
Public Utilities, General Disclosures [Line Items]
Deadband around base electric supply costs			$ 4.1
Customer sharing percentage of amounts exceeding deadband			90.00%
Company sharing percentage of amounts exceeding deadband			10.00%
Under collected supply costs		$ 23.8			$ 23.8
Proposed disallowance of replacement power costs		$ 6.0
Proposed under recovery due to statute change					$ 4.0
Customer sharing percentage of over/under collection of QF costs			100.00%
Regulatory assets associated with PCCAM	$ 2.1			2.1
Montana Accounting Order [Member]
Public Utilities, General Disclosures [Line Items]
Public Utilities, Request to Recognize Pension Contribution over 5 years	40.0
South Dakota Accounting Order [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory Asset for COVID Uncollectable Accounts	$ 0.4			$ 0.4