Property, Plant and Equipment (Details) (USD $)	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Public Utilities, Property, Plant and Equipment [Abstract]
Public Utilities, Property, Plant and Equipment, Disclosure of Composite Depreciation Rate for Plants in Service	2.80%	2.80%	2.80%
Weighted Average Depreciation Rates Non Utility	2.20%	2.20%	2.20%
Property, Plant and Equipment, Net, by Type [Abstract]
Utility plant in service	$ 2,992,560,000us-gaap_PropertyPlantAndEquipmentGross	$ 2,918,739,000us-gaap_PropertyPlantAndEquipmentGross
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	870,967,000	855,865,000
Total property, plant, and equipment, net	2,121,593,000us-gaap_PropertyPlantAndEquipmentNet	2,062,874,000us-gaap_PropertyPlantAndEquipmentNet
Property, Plant, Equipment non-cash	8,757,000nwn_PropertyPlantEquipmentnoncash	10,691,000nwn_PropertyPlantEquipmentnoncash
Equipment Expense	1,300,000us-gaap_EquipmentExpense	200,000us-gaap_EquipmentExpense
Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Utility plant in service	2,661,097,000	2,585,901,000
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	24,886,000	28,855,000
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	836,510,000	827,380,000
Total property, plant, and equipment, net	1,849,473,000	1,787,376,000
Non Utility Plant [Member]
Property, Plant and Equipment, Net, by Type [Abstract]
Utility plant in service	297,295,000	297,330,000
Public Utilities, Property, Plant and Equipment, Construction Work in Progress	9,282,000	6,653,000
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	34,457,000	28,485,000
Total property, plant, and equipment, net	$ 272,120,000	$ 275,498,000

Property, Plant, Equipment non-cash

No definition available.

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- Definition

The weighted average depreciation rates for non-utility plant

+ References

No definition available.

+ Details

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- Definition

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

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 210.5-02.14)

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

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 14

-Article 5

+ Details

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- Definition

This element represents equipment expense including depreciation, repairs, rentals, and service contract costs. This item also includes equipment purchases which do not qualify for capitalization in accordance with the entity's accounting policy. This item may also include furniture expenses.

+ References

No definition available.

+ Details

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- Definition

Amount before accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 50

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

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 210.5-02.13)

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

+ Details

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Balance Type:	debit
Period Type:	instant

- Definition

Amount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 360

-SubTopic 10

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

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 210.5-02.13)

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

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 13

-Subparagraph a

-Article 5

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 03

-Paragraph 8

-Article 7

+ Details

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- Details

Name:	us-gaap_PublicUtilitiesPropertyPlantAndEquipmentAbstract
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Balance Type:	na
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- Definition

Period end amount of construction work in progress in public utility.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Composite depreciation rate for public utility plants in service.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
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- Details

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