Employee Benefit Plans Investment Strategy (Details)	12 Months Ended
Dec. 31, 2011	Dec. 31, 2010
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Target asset allocation allowable range of plus or minus	5.00%
Pension Plans, Defined Benefit [Member] \| Domestic [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Debt securities	40.00%	40.00%
Equity securities	40.00%	40.00%
Pension Plans, Defined Benefit [Member] \| International [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Debt securities	10.00%	10.00%
Equity securities	10.00%	10.00%
Northwestern Energy Pension Plan Montana [Member]
Defined Benefit Plan, Actual Plan Asset Allocations [Abstract]
Cash and cash equivalents	0.00%	0.00%
Actual plan asset allocations	100.00%	100.00%
Northwestern Energy Pension Plan Montana [Member] \| Domestic [Member]
Defined Benefit Plan, Actual Plan Asset Allocations [Abstract]
Debt securities	39.50%	37.50%
Equity securities	40.30%	41.90%
Northwestern Energy Pension Plan Montana [Member] \| International [Member]
Defined Benefit Plan, Actual Plan Asset Allocations [Abstract]
Debt securities	10.60%	10.20%
Equity securities	9.60%	10.40%
Northwestern Pension Plan South Dakota [Member]
Defined Benefit Plan, Actual Plan Asset Allocations [Abstract]
Cash and cash equivalents	0.00%	0.00%
Actual plan asset allocations	100.00%	100.00%
Northwestern Pension Plan South Dakota [Member] \| Domestic [Member]
Defined Benefit Plan, Actual Plan Asset Allocations [Abstract]
Debt securities	38.40%	37.00%
Equity securities	40.90%	41.80%
Northwestern Pension Plan South Dakota [Member] \| International [Member]
Defined Benefit Plan, Actual Plan Asset Allocations [Abstract]
Debt securities	11.20%	10.50%
Equity securities	9.50%	10.70%
Northwestern Energy Health And Welfare [Member]
Defined Benefit Plan, Actual Plan Asset Allocations [Abstract]
Cash and cash equivalents	2.00%	0.00%
Actual plan asset allocations	100.00%	100.00%
Northwestern Energy Health And Welfare [Member] \| Domestic [Member]
Defined Benefit Plan, Actual Plan Asset Allocations [Abstract]
Debt securities	39.40%	39.10%
Equity securities	49.80%	50.70%
Northwestern Energy Health And Welfare [Member] \| International [Member]
Defined Benefit Plan, Actual Plan Asset Allocations [Abstract]
Debt securities	0.00%	0.00%
Equity securities	8.80%	10.20%
Other Postretirement Benefit Plans, Defined Benefit [Member] \| Domestic [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Debt securities	40.00%	40.00%
Equity securities	50.00%	50.00%
Other Postretirement Benefit Plans, Defined Benefit [Member] \| International [Member]
Defined Benefit Plan, Assets, Target Allocations [Abstract]
Debt securities	0.00%	0.00%
Equity securities	10.00%	10.00%

Name:	us-gaap_DefinedBenefitPlanAssetsTargetAllocationsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The percentage of the fair value of debt securities to the fair value of total plan assets held as of the measurement date.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph d(1)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 5

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e2446-114920

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (d)(1)

-URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e1928-114920

+ Details

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

- Definition

The percentage of the fair value of equity securities to the fair value of total plan assets held as of the measurement date.

+ References

+ Details

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

- Definition

The percentage of the fair value of investments other than debt securities, equity securities, and real estate to the fair value of total plan assets held as of the measurement date.

+ References

+ Details

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

- Details

Name:	us-gaap_DefinedBenefitPlansAndOtherPostretirementBenefitPlansTableTextBlockLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Target allocation percentage of investments in debt securities to total plan assets presented on a weighted-average basis as of the measurement date of the latest statement of financial position.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

-Subparagraph d(2)

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 5

-Subparagraph (c)(i)

-URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e2446-114920

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (d)(i)

-URI http://asc.fasb.org/extlink&oid=6962843&loc=d3e1928-114920

+ Details

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

- Definition

Target allocation percentage of investments in equity securities to total plan assets presented on a weighted-average basis as of the measurement date of the latest statement of financial position.

+ References

No definition available.

+ Details

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

- Definition

Target allocation maximum percentage of investments in equity securities to total plan assets presented on a weighted-average basis as of the measurement date of the latest statement of financial position.

+ References

No definition available.

+ Details

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

- Definition

The aggregate percentage of the fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of total plan assets held as of the measurement date.

+ References

+ Details

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

- Details

Name:	us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocationsAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration