Short-Term Borrowings and Credit Arrangements (Details) - USD ($)	12 Months Ended
Dec. 31, 2022	Dec. 31, 2021
Short-term Debt [Line Items]
Maximum borrowing capacity	$ 550,000,000.0	$ 450,000,000.0
Maximum Ratio Of Indebtedness To Net Capital Threshold Percentage	65.00%
Revolving Credit Facility [Member]
Short-term Debt [Line Items]
Commitment fees	$ 100,000	400,000
Swingline Credit Facility [Member]
Short-term Debt [Line Items]
Maximum borrowing capacity	$ 25,000,000.0	25,000,000.0
Line of credit facility, expiration date	Mar. 27, 2024
Unsecured Revolving Line Of Credit, Maximun Outstanding Under Accordian Feature [Member]
Short-term Debt [Line Items]
Maximum borrowing capacity	$ 75,000,000
Revolving Credit Facility Due 2024
Short-term Debt [Line Items]
Maximum borrowing capacity	$ 100,000,000.0	0
Line of credit facility, expiration date	Apr. 28, 2024
Revolving Credit Facility Due 2027
Short-term Debt [Line Items]
Maximum borrowing capacity	$ 425,000,000	$ 425,000,000.0
Line of credit facility, expiration date	May 18, 2027
Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate \| Swingline Credit Facility [Member]
Short-term Debt [Line Items]
Basis spread on variable rate	0.90%
Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate \| Unsecured Revolving Line of Credit
Short-term Debt [Line Items]
Basis spread on variable rate	0.10%
Measurement Input, Credit Spread [Member] \| Revolving Credit Facility [Member] \| Minimum [Member]
Short-term Debt [Line Items]
Basis spread on variable rate	1.00%
Measurement Input, Credit Spread [Member] \| Revolving Credit Facility [Member] \| Maximum [Member]
Short-term Debt [Line Items]
Basis spread on variable rate	1.75%
Base Rate \| Revolving Credit Facility [Member] \| Minimum [Member]
Short-term Debt [Line Items]
Basis spread on variable rate	0.00%
Base Rate \| Revolving Credit Facility [Member] \| Maximum [Member]
Short-term Debt [Line Items]
Basis spread on variable rate	0.75%
Base Rate \| Swingline Credit Facility [Member]
Short-term Debt [Line Items]
Basis spread on variable rate	0.125%

Maximum Ratio Of Indebtedness To Net Capital Threshold Percentage

No definition available.

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Namespace Prefix:	nwe_
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Period Type:	duration

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of the fee for available but unused credit capacity under the credit facility.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
-URI https://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

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Balance Type:	debit
Period Type:	duration

- Definition

Date the credit facility terminates, in YYYY-MM-DD format.

+ References

+ Details

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Period Type:	duration

- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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