Short-Term Borrowings and Credit Arrangements (Details) - USD ($)	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 600,000,000.0	$ 550,000,000.0
Maximum Ratio Of Indebtedness To Net Capital Threshold Percentage	65.00%
Short-Term Debt	$ 100,000,000	0
Term Loan Due 2025
Line of Credit Facility [Line Items]
Maximum Ratio Of Indebtedness To Net Capital Threshold Percentage	65.00%
Maturity date	Apr. 11, 2025
Short-Term Debt	$ 100,000,000.0
Revolving Credit Facility Due 2028 [Member] | NWE Public Service
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	200,000,000.0
Credit Facility Increase to Maximum Borrowing Capacity	50,000,000
Line Of Credit Facility Base Sublimit	150,000,000.0
Revolving Credit Facility Due 2028 [Member] | NorthWestern Energy Group, Inc.
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	200,000,000.0
Credit Facility Increase to Maximum Borrowing Capacity	50,000,000
Line Of Credit Facility Base Sublimit	50,000,000.0
Revolving Credit Facility Due 2028 [Member] | NW Corp
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 600,000,000.0	425,000,000.0
Line of credit facility, expiration date	Nov. 29, 2028
Credit Facility Increase to Maximum Borrowing Capacity	$ 100,000,000.0
Revolving Credit Facility Due 2028 [Member] | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate | NWE Public Service
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.10%
Revolving Credit Facility Due 2028 [Member] | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate | NorthWestern Energy Group, Inc.
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.10%
Revolving Credit Facility Due 2028 [Member] | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate | NW Corp
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.10%
Revolving Credit Facility Due 2028 [Member] | Measurement Input, Credit Spread [Member] | Minimum [Member] | NWE Public Service
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.00%
Revolving Credit Facility Due 2028 [Member] | Measurement Input, Credit Spread [Member] | Minimum [Member] | NorthWestern Energy Group, Inc.
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.00%
Revolving Credit Facility Due 2028 [Member] | Measurement Input, Credit Spread [Member] | Minimum [Member] | NW Corp
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.00%
Revolving Credit Facility Due 2028 [Member] | Measurement Input, Credit Spread [Member] | Maximum [Member] | NWE Public Service
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.75%
Revolving Credit Facility Due 2028 [Member] | Measurement Input, Credit Spread [Member] | Maximum [Member] | NorthWestern Energy Group, Inc.
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.75%
Revolving Credit Facility Due 2028 [Member] | Measurement Input, Credit Spread [Member] | Maximum [Member] | NW Corp
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.75%
Revolving Credit Facility Due 2028 [Member] | Base Rate | Minimum [Member] | NWE Public Service
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.00%
Revolving Credit Facility Due 2028 [Member] | Base Rate | Minimum [Member] | NorthWestern Energy Group, Inc.
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.00%
Revolving Credit Facility Due 2028 [Member] | Base Rate | Minimum [Member] | NW Corp
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.00%
Revolving Credit Facility Due 2028 [Member] | Base Rate | Maximum [Member] | NWE Public Service
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.75%
Revolving Credit Facility Due 2028 [Member] | Base Rate | Maximum [Member] | NorthWestern Energy Group, Inc.
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.75%
Revolving Credit Facility Due 2028 [Member] | Base Rate | Maximum [Member] | NW Corp
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.75%
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Commitment fees	$ 700,000	600,000
Swingline Credit Facility [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 0	25,000,000.0
Line of credit facility, expiration date	Mar. 27, 2025
Swingline Credit Facility [Member] | Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.90%
Swingline Credit Facility [Member] | Base Rate
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.125%
Revolving Credit Facility Due 2024
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 0	$ 100,000,000.0
Revolving Credit Facility Due 2028 - Effective January 1, 2024 | NWE Public Service
Line of Credit Facility [Line Items]
Maximum borrowing capacity	200,000,000.0
Revolving Credit Facility Due 2028 - Effective January 1, 2024 | NorthWestern Energy Group, Inc.
Line of Credit Facility [Line Items]
Maximum borrowing capacity	100,000,000.0
Revolving Credit Facility Due 2028 - Effective January 1, 2024 | NW Corp
Line of Credit Facility [Line Items]
Maximum borrowing capacity	400,000,000.0
Northwestern Corporation Revolving Credit Facility Terminated 2024
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 100,000,000.0