Income Tax (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Income Tax Expense [Abstract]
Income tax charge from increased effective Oregon tax rate		$ 600
Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount	18,892	25,721
Differences Required To Be Flowed Through By Regulatory Commissions	1,329nwn_DifferencesRegulatoryCommission	1,433nwn_DifferencesRegulatoryCommission
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount	(1,138)	(169)
Income Tax Expense (Benefit)	$ 19,083us-gaap_IncomeTaxExpenseBenefit	$ 26,985us-gaap_IncomeTaxExpenseBenefit
Effective Income Tax Rate	40.10%	41.60%

Tax differences flowed-through by regualtory commissions

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Income tax charge from increased effective Oregon tax rate

+ References

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

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-Name Accounting Standards Codification

-Topic 235

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-Section S99

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

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-Glossary Income Tax Expense (or Benefit)

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The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from continuing operations.

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-Publisher FASB

-Name Accounting Standards Codification

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to other adjustments.

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