Long-Term Debt and Capital Leases Schedule of Debt (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Debt Instrument [Line Items]
Long-term debt	$ 1,662,099us-gaap_LongTermDebt	$ 1,155,097us-gaap_LongTermDebt
Less current maturities	0us-gaap_LongTermDebtCurrent	0us-gaap_LongTermDebtCurrent
Long-term debt, excluding current maturities	1,662,099us-gaap_LongTermDebtNoncurrent	1,155,097us-gaap_LongTermDebtNoncurrent
Capital Lease
Total Capital Leases	29,892us-gaap_CapitalLeaseObligations	31,557us-gaap_CapitalLeaseObligations
Less current maturities	(1,730)us-gaap_CapitalLeaseObligationsCurrent	(1,662)us-gaap_CapitalLeaseObligationsCurrent
Capital lease obligations, noncurrent	28,162	29,895
Unsecured Debt \| Unsecured Revolving Line Of Credit
Debt Instrument [Line Items]
Long-term debt	0	0
Maturity date	Nov. 05, 2018
Secured Debt \| South Dakota, 6.05%, Due 2018
Debt Instrument [Line Items]
Long-term debt	55,000	55,000
Interest rate, stated percentage	6.05%
Maturity date	May 01, 2018
Secured Debt \| South Dakota, 5.01%, Due 2025
Debt Instrument [Line Items]
Long-term debt	64,000	64,000
Interest rate, stated percentage	5.01%
Maturity date	May 01, 2025
Secured Debt \| South Dakota, 4.15%, Due 2042
Debt Instrument [Line Items]
Long-term debt	30,000	30,000
Interest rate, stated percentage	4.15%
Maturity date	Aug. 10, 2042
Secured Debt \| South Dakota, 4.30%, Due 2052
Debt Instrument [Line Items]
Long-term debt	20,000	20,000
Interest rate, stated percentage	4.30%
Maturity date	Aug. 10, 2052
Secured Debt \| South Dakota, 4.85% Due 2043
Debt Instrument [Line Items]
Long-term debt	50,000	50,000
Interest rate, stated percentage	4.85%
Maturity date	Dec. 19, 2043
Secured Debt \| Secured Debt South Dakota Due 2044 [Member]
Debt Instrument [Line Items]
Long-term debt	30,000	0
Interest rate, stated percentage	4.22%
Maturity date	Dec. 19, 2044
Secured Debt \| Montana, 6.04%, Due 2016
Debt Instrument [Line Items]
Long-term debt	150,000	150,000
Interest rate, stated percentage	6.04%
Maturity date	Sep. 01, 2016
Secured Debt \| Montana, 6.34%, Due 2019
Debt Instrument [Line Items]
Long-term debt	250,000	250,000
Interest rate, stated percentage	6.34%
Maturity date	Apr. 01, 2019
Secured Debt \| Montana, 5.71%, Due 2039
Debt Instrument [Line Items]
Long-term debt	55,000	55,000
Interest rate, stated percentage	5.71%
Maturity date	Oct. 15, 2039
Secured Debt \| Montana, 5.01%, Due 2025
Debt Instrument [Line Items]
Long-term debt	161,000	161,000
Interest rate, stated percentage	5.01%
Maturity date	May 01, 2025
Secured Debt \| Montana, 4.15%, Due 2042
Debt Instrument [Line Items]
Long-term debt	60,000	60,000
Interest rate, stated percentage	4.15%
Maturity date	Aug. 10, 2042
Secured Debt \| Montana, 4.30%, Due 2052
Debt Instrument [Line Items]
Long-term debt	40,000	40,000
Interest rate, stated percentage	4.30%
Maturity date	Aug. 10, 2052
Secured Debt \| Montana 4.85%, Due 2043
Debt Instrument [Line Items]
Long-term debt	15,000	15,000
Interest rate, stated percentage	4.85%
Maturity date	Dec. 19, 2043
Secured Debt \| Montana 3.99% Due 2028
Debt Instrument [Line Items]
Long-term debt	35,000	35,000
Interest rate, stated percentage	3.99%
Maturity date	Dec. 19, 2028
Secured Debt \| Montana 4.176% Due 2044 [Member]
Debt Instrument [Line Items]
Long-term debt	450,000	0
Interest rate, stated percentage	4.176%
Maturity date	Nov. 15, 2044
Secured Debt \| Montana, 4.65%, Due 2023
Debt Instrument [Line Items]
Long-term debt	170,205	170,205
Interest rate, stated percentage	4.65%
Maturity date	Aug. 01, 2023
Secured Debt \| New Market Tax Credit Financing [Member]
Debt Instrument [Line Items]
Long-term debt	26,977	0
Interest rate, stated percentage	1.146%
Maturity date	Jul. 01, 2046
Unamortized Discount Premium, Net \| Discount on Notes and Bonds
Debt Instrument [Line Items]
Long-term debt	$ (83)	$ (108)

Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date.

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

- Details

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

- Definition

Amount of capital lease obligation due within one year or the normal operating cycle, if longer.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19

-Article 5

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.20)

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

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 30

-Section 50

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6455314&loc=d3e45023-112735

+ Details

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

- Definition

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 30

-Section 50

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6455398&loc=d3e45280-112737

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 30

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6455314&loc=d3e45023-112735

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22)

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

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

+ Details

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(1))

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

+ Details

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

- Details

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

- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 22

-Article 5

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22(a)(2))

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

+ Details

Name:	us-gaap_DebtInstrumentMaturityDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

-Article 7

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.22)

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

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-03.16)

-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 16

-Article 9

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 20, 22

-Article 5

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 210

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-03.16)

-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

+ Details

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

- Definition

Amount of long-term debt, after unamortized discount or premium, scheduled to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ 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 210.5-02.19,20)

-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 20

-Article 5

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19

-Article 5

+ Details

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

- Definition

Carrying amount of long-term debt, net of unamortized discount or premium, excluding amounts to be repaid within one year or the normal operating cycle, if longer (current maturities). Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ 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 210.5-02.22)

-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 22

-Article 5

+ Details

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

- Details

Name:	us-gaap_LongtermDebtTypeAxis=nwe_DebtInstrumentUnamortizedDiscountPremiumNetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_LineOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SecuredDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: