Estimated capital expenditure projections related to environmental obligations.

No definition available.

Name:	nwe_EnvironmentalObligationEstimatedCapitalExpenditures
Namespace Prefix:	nwe_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Environmental Remediation Obligation Maximum

+ References

No definition available.

+ Details

Name:	nwe_EnvironmentalRemediationObligationMaximum
Namespace Prefix:	nwe_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Environmental Remediation Obligation Minimum

+ References

No definition available.

+ Details

Name:	nwe_EnvironmentalRemediationObligationMinimum
Namespace Prefix:	nwe_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Environmental Remediation Obligation, To be Incurred During Next 5 years

+ References

No definition available.

+ Details

Name:	nwe_EnvironmentalRemediationObligationToBeIncurredDuringNext5Years
Namespace Prefix:	nwe_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

NOx Emissions Per Million Btu As Calculated On A 30-day Rolling Basis

+ References

No definition available.

+ Details

Name:	nwe_NOxEmissionsPerMillionBtuAsCalculatedOnA30dayRollingBasis
Namespace Prefix:	nwe_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

- Definition

Number Of Years For Environmental Remediation Obligation To Be Incurred

+ References

No definition available.

+ Details

Name:	nwe_NumberOfYearsForEnvironmentalRemediationObligationToBeIncurred
Namespace Prefix:	nwe_
Data Type:	us-types:durationStringItemType
Balance Type:	na
Period Type:	duration

- Definition

Total costs accrued as of the balance sheet date for environmental loss contingencies.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 30
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13207-110859

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section 25
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6395460&loc=d3e13647-108346

+ Details

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

- Definition

The proportionate share of interest (percent) in a jointly owned utility plant.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 980
-SubTopic 360
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 10.C)
-URI http://asc.fasb.org/extlink&oid=6500719&loc=d3e659983-123033

+ Details

Name:	us-gaap_JointlyOwnedUtilityPlantProportionateOwnershipShare
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Minimum amount of other commitment not otherwise specified in the taxonomy. Excludes commitments explicitly modeled in the taxonomy, including but not limited to, long-term and short-term purchase commitments, recorded and unrecorded purchase obligations, supply commitments, registration payment arrangements, leases, debt, product warranties, guarantees, environmental remediation obligations, and pensions.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_JointlyOwnedUtilityPlantAxis=nwe_ColstripUnit4Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_JointlyOwnedUtilityPlantAxis=nwe_CoyoteGeneratingFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_EnvironmentalRemediationContingencyAxis=nwe_EnvironmentalRemediationObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_EnvironmentalRemediationSiteAxis=nwe_AberdeenSouthDakotaSiteMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_EnvironmentalRemediationContingencyAxis=nwe_ManufacturedGasPlantsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_EnvironmentalRemediationSiteAxis=nwe_CombinedManufacturingSitesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Commitments and Contingencies Environmental Liabilities (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)	Jul. 01, 2018	Dec. 31, 2015
Other Commitment	$ 22.0
Colstrip Unit 4 [Member]
Environmental obligation, estimated capital expenditures	$ 90.0
Jointly owned utility plant, proportionate ownership share	30.00%		30.00%
Coyote Generating Facility [Member]
Jointly owned utility plant, proportionate ownership share	10.00%		10.00%
Environmental remediation obligations [Member]
Environmental remediation obligation, minimum	$ 27.9
Environmental remediation obligation, maximum	32.6
Accrual for environmental loss contingencies	31.5
Scenario, Forecast [Member] \| Coyote Generating Facility [Member]
NOx emissions per million Btu		0.5
Aberdeen South Dakota Site [Member] \| Manufactured Gas Plants [Member]
Accrual for environmental loss contingencies	10.8
Environmental remediation obligation, to be incurred during next 5 years	$ 6.2
Number of years for environmental remediation obligation to be incurred	5 years
Combined Manufacturing Sites [Member] \| Manufactured Gas Plants [Member]
Accrual for environmental loss contingencies	$ 24.7