2.4.0.82407402 - Disclosure - Stock-Based Compensation (details)truefalsefalse1false USDfalsefalse$D2013Q2http://www.sec.gov/CIK0000073020duration2013-04-01T00:00:002013-06-30T00:00:00usdPerShareDividehttp://www.xbrl.org/2003/iso4217USDiso4217http://www.xbrl.org/2003/instancesharesxbrli0usdStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$2false USDfalsefalseD2013Q2YTDhttp://www.sec.gov/CIK0000073020duration2013-01-01T00:00:002013-06-30T00:00:00numberStandardhttp://www.xbrl.org/2003/instancepurexbrli0sharesStandardhttp://www.xbrl.org/2003/instancesharesxbrli0usdPerShareDividehttp://www.xbrl.org/2003/iso4217USDiso4217http://www.xbrl.org/2003/instancesharesxbrli0usdStandardhttp://www.xbrl.org/2003/iso4217USDiso42170$1true 1nwn_LtipParagraph1Abstractnwn_falsenadurationfalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringfalse0LTIP Paragraph 1 [Abstract]2false 2us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPeriodIncreaseDecreaseus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1falsefalsefalse00falsefalsefalse2truefalsefalse3730037300falsefalsefalsexbrli:sharesItemTypesharesThe net total number of shares (or other type of equity) under an equity-based award plan, other than a stock option plan, that were granted, vested and forfeited during the reporting period.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 false1Common stock was authorized for grants under the LTIP3false 2us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueus-gaap_truenainstantfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1truefalsefalse38.9638.96USD$falsetruefalse2truefalsefalse38.9638.96USD$falsetruefalsenum:perShareItemTypedecimalThe weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(i)-(ii) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 false3Weighted-average per share grant date fair value of unvested shares4false 2us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsMethodUsedus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1falsefalsefalse00falsefalsefalse2falsefalsefalse00Monte-Carlo option pricing modelfalsefalsefalsexbrli:stringItemTypestringFor each plan, identification of the award pricing model or other valuation method used in calculating the weighted average fair values disclosed. The model is also used to calculate the compensation expense that is shown within the balance sheet, income statement, and cash flow. Examples of valuation techniques are lattice models (binomial model), closed-form models (Black-Scholes-Merton formula), and a Monte Carlo simulation technique. Fair value is the amount at which an asset or liability could be bought or incurred or sold or settled in a current transaction between willing parties, that is, other than in a forced or liquidation sale. May include disclosures about the assumptions underlying application of the method selected.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(1) -URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901 false0Fair value pricing model LTIP5false 2nwn_ShareBasedCompensationArrangementByShareBasedPaymentAwardMarketValueOnValuationDatenwn_falsenainstantfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1truefalsefalse45.3845.38USD$falsetruefalse2truefalsefalse45.3845.38USD$falsetruefalsenum:perShareItemTypedecimalStock price on the Long Term Incentive Plan (LTIP) grant dateNo definition available.false3Stock price on valuation date6false 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