Short-Term Borrowings and Credit Arrangements Unsecured Revolving Line of Credit (Details) (Unsecured Revolving Line Of Credit [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014 numberofbanks	Dec. 31, 2013
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 350
Number of institutions participating in the credit facility	8
Maximum number of institutions participating in the credit faciltiy pertaining to maximum contributory percentage	1
Line of Credit Facility, maximum percentage of total availability provided by a single lender	21.00%
Commitment fees	0.4	0.5
Letters of credit outstanding, amount	$ 0
Maximum ratio of indebtedness to net capital threshold percentage	65.00%
Eurodollar [Member] \| Minimum [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.88%
Eurodollar [Member] \| Maximum [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	1.75%
Base Rate [Member] \| Minimum [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.00%
Base Rate [Member] \| Maximum [Member]
Line of Credit Facility [Line Items]
Basis spread on variable rate	0.75%

Maximum percentage of total availability provided by a single lender for the line of credit facility.

No definition available.

Name:	nwe_LineOfCreditFaciiityMaximumPercentageOfTotalAvailabilityProvidedBySingleLender
Namespace Prefix:	nwe_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line Of Credit Facility, Maximum Number Of Institutions Participating In The Faciltiy Pertaining To Maximum Contributory Percentage

+ References

No definition available.

+ Details

Name:	nwe_LineOfCreditFacilityMaximumNumberOfInstitutionsParticipatingInFaciltiyPertainingToMaximumContributoryPercentage
Namespace Prefix:	nwe_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Line Of Credit Facility, Number Of Institutions Participating In The Faciltiy

+ References

No definition available.

+ Details

Name:	nwe_LineOfCreditFacilityNumberOfInstitutionsParticipatingInFaciltiy
Namespace Prefix:	nwe_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Maximum Ratio Of Indebtedness To Net Capital Threshold Percentage

+ References

No definition available.

+ Details

Name:	nwe_MaximumRatioOfIndebtednessToNetCapitalThresholdPercentage
Namespace Prefix:	nwe_
Data Type:	num:percentItemType
Balance Type:	na
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

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

+ Details

Name:	us-gaap_LineOfCreditFacilityCommitmentFeeAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_LineOfCreditFacilityLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
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

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant