Commitments and Contingencies Qualifying Facilities Liability (Details) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Ending QF liability	$ 136,893,000nwe_QualifyingFacilityLiability	$ 136,448,000nwe_QualifyingFacilityLiability
Qualifying Facility Contracts [Member]
Beginning QF liability	136,448,000	136,652,000
Unrecovered amount	(10,128,000)	(10,647,000)
Interest expense	10,573,000	10,443,000
Ending QF liability	136,893,000	136,448,000
Qualifying Facility Contracts [Member] \| Gross Obligation [Member]
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2015	69,606,000
2016	71,598,000
2017	73,622,000
2018	75,688,000
2019	77,791,000
Thereafter	646,783,000
Contractual obligation related to QF's	1,015,088,000
Qualifying Facility Contracts [Member] \| Recoverable Amounts [Member]
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2015	56,598,000
2016	57,188,000
2017	57,789,000
2018	58,401,000
2019	59,020,000
Thereafter	508,195,000
Contractual obligation related to QF's	797,191,000
Qualifying Facility Contracts [Member] \| Net Amount [Member]
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
2015	13,008,000
2016	14,410,000
2017	15,833,000
2018	17,287,000
2019	18,771,000
Thereafter	138,588,000
Contractual obligation related to QF's	217,897,000
Qualifying Facility Contracts [Member] \| Year 2029 [Member] \| Gross Obligation [Member]
Recorded unconditional purchase obligation portion recoverable through rates	800,000,000
Recorded Unconditional Purchase Obligation, Fiscal Year Maturity Schedule [Abstract]
Contractual obligation related to QF's	1,000,000,000
Qualifying Facility Contracts [Member] \| Minimum [Member] \| Year 2029 [Member]
Price per MWH of energy required to be purchased per QF agreement	74
Qualifying Facility Contracts [Member] \| Maximum [Member] \| Year 2029 [Member]
Price per MWH of energy required to be purchased per QF agreement	$ 136

Price Per MWH Of Energy Required To Be Purchase Per QF Agreement

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- Definition

Qualifying Facility Liability

+ References

No definition available.

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- Definition

Recorded Unconditional Purchase Obligation Portion Recoverable Through Rates

+ References

No definition available.

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- Definition

Unrecovered Liability Amount

+ References

No definition available.

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- Definition

Amount of the cost of borrowed funds accounted for as interest expense.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 835

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6450988&loc=d3e26243-108391

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.9)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 9

-Article 9

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- Definition

Amount of the recorded obligation to transfer funds in the future for fixed or minimum amounts or quantities of goods or services at fixed or minimum prices (for example, as in take-or-pay contracts or throughput contracts).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 440

-SubTopic 10

-Section 50

-Paragraph 6

-URI http://asc.fasb.org/extlink&oid=6394976&loc=d3e25383-109308

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Amount of the recorded unconditional purchase obligation maturing after the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

+ References

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Amount of the recorded unconditional purchase obligation maturing in the fifth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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- Definition

Amount of the recorded unconditional purchase obligation maturing in the fourth fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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- Definition

Amount of the recorded unconditional purchase obligation maturing in the second fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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- Definition

Amount of the recorded unconditional purchase obligation maturing in the third fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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Amount of the recorded unconditional purchase obligation maturing in the next fiscal year following the latest fiscal year. Excludes interim and annual periods when interim periods are reported on a rolling approach, from latest balance sheet date.

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