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Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 true2Defined Benefit Plan Net Periodic Benefit Cost23false 4nwn_DefinedBenefitPlanAmountAllocatedToConstructionnwn_falsedebitdurationfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1truefalsefalse-1801000-1801000USD$falsefalsefalse2truefalsefalse-1428000-1428000USD$falsefalsefalse3truefalsefalse-3656000-3656000USD$falsefalsefalse4truefalsefalse-2846000-2846000USD$falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalsexbrli:monetaryItemTypemonetaryNet periodic benefit cost allocated to construction.No definition available.false2Defined Benefit Plan Amount Allocated To Construction24false 4nwn_DefinedBenefitPlanNetAmountDeferredToRegulatoryBalancingAccountnwn_falsedebitdurationfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1truefalsefalse-2271000-2271000[1]USD$falsefalsefalse2truefalsefalse-2094000-2094000[1]USD$falsefalsefalse3truefalsefalse-4620000-4620000[1]USD$falsefalsefalse4truefalsefalse-4162000-4162000[1]USD$falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalsexbrli:monetaryItemTypemonetaryNet periodic benefit cost deferred to regulatory balancing account.No definition available.false2Defined Benefit Plan Net Amount Deferred To Regulatory Balancing Account25false 4nwn_DefinedBenefitPlanNetAmountChargedToExpensenwn_falsedebitdurationfalsefalsefalsefalsefalsefalsefalsefalsetotalLabel1truefalsefalse21710002171000USD$falsefalsefalse2truefalsefalse21660002166000USD$falsefalsefalse3truefalsefalse42100004210000USD$falsefalsefalse4truefalsefalse43680004368000USD$falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalsexbrli:monetaryItemTypemonetaryNet periodic benefit cost charged to expense.No definition available.true2Defined Benefit Plan Net Amount Charged To Expense26false 0truefalsetruefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalse11false USDtruefalse$D2013Q2_us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis_us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMemberhttp://www.sec.gov/CIK0000073020duration2013-04-01T00:00:002013-06-30T00:00:00falsefalseOther Postretirement Benefit Plans, Defined Benefit [Member]us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxisxbrldihttp://xbrl.org/2006/xbrldius-gaap_OtherPostretirementBenefitPlansDefinedBenefitMemberus-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxisexplicitMemberusdStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$nanafalse0Other Postretirement Benefit Plans, Defined Benefit [Member]27true 3us-gaap_DefinedBenefitPlansAndOtherPostretirementBenefitPlansDisclosuresAbstractus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringfalse0Defined Benefit Plans and Other Postretirement Benefit Plans Disclosures [Abstract]28false 4us-gaap_DefinedBenefitPlanServiceCostus-gaap_truedebitdurationfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1truefalsefalse179000179000USD$falsefalsefalse2truefalsefalse177000177000USD$falsefalsefalse3truefalsefalse358000358000USD$falsefalsefalse4truefalsefalse354000354000USD$falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalsexbrli:monetaryItemTypemonetaryThe actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the plan.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Service Cost (Component of Net Periodic Pension Cost) -URI http://asc.fasb.org/extlink&oid=6525008 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 false2Defined Benefit Plan, Service Cost29false 4us-gaap_DefinedBenefitPlanInterestCostus-gaap_truedebitdurationfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1truefalsefalse286000286000USD$falsefalsefalse2truefalsefalse315000315000USD$falsefalsefalse3truefalsefalse572000572000USD$falsefalsefalse4truefalsefalse629000629000USD$falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalsexbrli:monetaryItemTypemonetaryThe increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(2) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 false2Defined Benefit Plan, Interest Cost30false 4us-gaap_DefinedBenefitPlanExpectedReturnOnPlanAssetsus-gaap_truecreditdurationfalsefalsefalsefalsefalsefalsefalsetruenegatedLabel1truefalsefalse00USD$falsefalsefalse2truefalsefalse00USD$falsefalsefalse3truefalsefalse00USD$falsefalsefalse4truefalsefalse00USD$falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalsexbrli:monetaryItemTypemonetaryAn amount calculated as a basis for determining the extent of delayed recognition of the effects of changes in the fair value of assets. The expected return on plan assets is determined based on the expected long-term rate of return on plan assets and the market-related value of plan assets.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 30 -Glossary Expected Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512136 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 60 -Glossary Expected Return on Plan Assets -URI http://asc.fasb.org/extlink&oid=6512171 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(3) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 false2Defined Benefit Plan Expected Return On Plan Assets31false 4us-gaap_DefinedBenefitPlanFutureAmortizationOfGainLossus-gaap_truecreditdurationfalsefalsefalsefalsefalsefalsefalsetruenegatedTerseLabel1truefalsefalse169000169000USD$falsefalsefalse2truefalsefalse103000103000USD$falsefalsefalse3truefalsefalse338000338000USD$falsefalsefalse4truefalsefalse206000206000USD$falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalsexbrli:monetaryItemTypemonetaryAmount included in accumulated other comprehensive income (AOCI) related to gain (loss) expected to be recognized as components of net periodic benefit cost for the fiscal year that follows the most recent annual statement of financial position.No definition available.false2Defined Benefit Plan Amortization Of Net Actuarial Loss32false 4us-gaap_DefinedBenefitPlanAmortizationOfPriorServiceCostCreditus-gaap_truedebitdurationfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1truefalsefalse4900049000USD$falsefalsefalse2truefalsefalse4900049000USD$falsefalsefalse3truefalsefalse9800098000USD$falsefalsefalse4truefalsefalse9800098000USD$falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalsexbrli:monetaryItemTypemonetaryThe amount of the prior service cost or credit recognized in net periodic benefit cost relating to benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 false2Defined Benefit Plan, Amortization of Prior Service Cost (Credit)33false 4us-gaap_DefinedBenefitPlanAmortizationOfTransitionObligationsAssetsus-gaap_truedebitdurationfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1truefalsefalse00USD$falsefalsefalse2truefalsefalse103000103000USD$falsefalsefalse3truefalsefalse00USD$falsefalsefalse4truefalsefalse206000206000USD$falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalsexbrli:monetaryItemTypemonetaryThe amount of the transition obligation or asset recognized in net periodic benefit cost.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h)(6) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 false2Defined Benefit Plan Amortization Of Transition Obligations34false 4us-gaap_DefinedBenefitPlanNetPeriodicBenefitCostus-gaap_truedebitdurationfalsefalsefalsefalsefalsefalsefalsefalsetotalLabel1truefalsefalse683000683000USD$falsefalsefalse2truefalsefalse747000747000USD$falsefalsefalse3truefalsefalse13660001366000USD$falsefalsefalse4truefalsefalse14930001493000USD$falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalsexbrli:monetaryItemTypemonetaryThe total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 true2Defined Benefit Plan Net Periodic Benefit Cost35false 4nwn_DefinedBenefitPlanAmountAllocatedToConstructionnwn_falsedebitdurationfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1truefalsefalse-211000-211000USD$falsefalsefalse2truefalsefalse-215000-215000USD$falsefalsefalse3truefalsefalse-430000-430000USD$falsefalsefalse4truefalsefalse-429000-429000USD$falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalsexbrli:monetaryItemTypemonetaryNet periodic benefit cost allocated to construction.No definition available.false2Defined Benefit Plan Amount Allocated To Construction36false 4nwn_DefinedBenefitPlanNetAmountDeferredToRegulatoryBalancingAccountnwn_falsedebitdurationfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1truefalsefalse00[1]USD$falsefalsefalse2truefalsefalse00[1]USD$falsefalsefalse3truefalsefalse00[1]USD$falsefalsefalse4truefalsefalse00[1]USD$falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalsexbrli:monetaryItemTypemonetaryNet periodic benefit cost deferred to regulatory balancing account.No definition available.false2Defined Benefit Plan Net Amount Deferred To Regulatory Balancing Account37false 4nwn_DefinedBenefitPlanNetAmountChargedToExpensenwn_falsedebitdurationfalsefalsefalsefalsefalsefalsefalsefalsetotalLabel1truefalsefalse472000472000USD$falsetruefalse2truefalsefalse532000532000USD$falsetruefalse3truefalsefalse936000936000USD$falsetruefalse4truefalsefalse10640001064000USD$falsetruefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalsexbrli:monetaryItemTypemonetaryNet periodic benefit cost charged to expense.No definition available.true2Defined Benefit Plan Net Amount Charged To Expense1Effective January 1, 2011, the OPUC approved the deferral of certain pension expenses above or below the amount set in rates, with recovery of these deferred amounts through the implementation of a balancing account, which includes the expectation of lower net periodic benefit costs in future years. Deferred pension expense balances accrue interest at the utility’s actual cost of long-term debt. See Note 2.falsePension and Other Postretirement Benefits (details) (USD $)NoRoundingUnKnownUnKnownUnKnowntruefalsefalseSheethttp://www.nwnatural.com/role/PensionAndOtherPostretirementBenefitsDetails637