2.4.0.82402402 - Disclosure - Summary of Significant Accounting Policies Regulatory Assets Disclosure (Details)truefalseIn Thousands, unless otherwise specifiedfalse1false USDfalsefalse$I2013Q2http://www.sec.gov/CIK0000073020instant2013-06-30T00:00:000001-01-01T00:00:00usdStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$2false USDfalsefalse$I2012Q4SDhttp://www.sec.gov/CIK0000073020instant2012-12-31T00:00:000001-01-01T00:00:00usdStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$3false USDfalsefalse$I2012Q3http://www.sec.gov/CIK0000073020instant2012-09-30T00:00:000001-01-01T00:00:00usdStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$4false USDfalsefalse$I2012Q2http://www.sec.gov/CIK0000073020instant2012-06-30T00:00:000001-01-01T00:00:00usdStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$5false USDfalsefalse$I2012Q1SD2http://www.sec.gov/CIK0000073020instant2012-03-31T00:00:000001-01-01T00:00:00usdStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$6false USDfalsefalse$I2011Q4http://www.sec.gov/CIK0000073020instant2011-12-31T00:00:000001-01-01T00:00:00usdStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$7false USDfalsefalse$I2011Q3http://www.sec.gov/CIK0000073020instant2011-09-30T00:00:000001-01-01T00:00:00usdStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$8false USDfalsefalse$I2011Q2http://www.sec.gov/CIK0000073020instant2011-06-30T00:00:000001-01-01T00:00:00usdStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$9false USDfalsefalse$I2011Q1QTDhttp://www.sec.gov/CIK0000073020instant2011-03-31T00:00:000001-01-01T00:00:00usdStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$1true 3us-gaap_RegulatoryAssetLineItemsus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalse7falsefalsefalse00falsefalsefalse8falsefalsefalse00falsefalsefalse9falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringfalse0Regulatory Assets [Line Items]2false 4us-gaap_RegulatoryAssetsNoncurrentus-gaap_truedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse393652000393652USD$falsetruefalse2truefalsefalse382255000382255USD$falsetruefalse3truefalsefalse362472000362472USD$falsetruefalse4truefalsefalse362290000362290USD$falsetruefalse5truefalsefalse364132000364132USD$falsetruefalse6truefalsefalse367536000367536USD$falsetruefalse7truefalsefalse325692000325692USD$falsetruefalse8truefalsefalse323371000323371USD$falsetruefalse9truefalsefalse343085000343085USD$falsetruefalsexbrli:monetaryItemTypemonetaryCarrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 false2Regulatory Assets, Noncurrent3false 4us-gaap_RegulatoryAssetsCurrentus-gaap_truedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse2595200025952USD$falsefalsefalse2truefalsefalse5244800052448USD$falsefalsefalse3falsefalsefalse00falsefalsefalse4truefalsefalse6529700065297USD$falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalse7falsefalsefalse00falsefalsefalse8falsefalsefalse00falsefalsefalse9falsefalsefalse00falsefalsefalsexbrli:monetaryItemTypemonetaryCarrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 false2Regulatory Assets, Current4false 0truefalsetruefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalse7falsefalsefalse00falsefalsefalse8falsefalsefalse00falsefalsefalse9falsefalsefalse00falsefalsefalse10false USDtruefalse$I2013Q2_us-gaap_RegulatoryAssetAxis_nwn_UnrealizedLossOnDerivativesMemberhttp://www.sec.gov/CIK0000073020instant2013-06-30T00:00:000001-01-01T00:00:00falsefalseUnrealized Loss On Derivatives [Member]us-gaap_RegulatoryAssetAxisxbrldihttp://xbrl.org/2006/xbrldinwn_UnrealizedLossOnDerivativesMemberus-gaap_RegulatoryAssetAxisexplicitMemberusdStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$nanafalse0Unrealized Loss On Derivatives [Member]5true 3us-gaap_RegulatoryAssetLineItemsus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalse7falsefalsefalse00falsefalsefalse8falsefalsefalse00falsefalsefalse9falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringfalse0Regulatory Assets [Line Items]6false 4us-gaap_RegulatoryAssetsNoncurrentus-gaap_truedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse17540001754[1]USD$falsefalsefalse2truefalsefalse578000578[1]USD$falsefalsefalse3falsefalsefalse00falsefalsefalse4truefalsefalse21300002130[1]USD$falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalse7falsefalsefalse00falsefalsefalse8falsefalsefalse00falsefalsefalse9falsefalsefalse00falsefalsefalsexbrli:monetaryItemTypemonetaryCarrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 false2Regulatory Assets, Noncurrent7false 4us-gaap_RegulatoryAssetsCurrentus-gaap_truedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse93920009392[1]USD$falsefalsefalse2truefalsefalse1079600010796[1]USD$falsefalsefalse3falsefalsefalse00falsefalsefalse4truefalsefalse2940700029407[1]USD$falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalse7falsefalsefalse00falsefalsefalse8falsefalsefalse00falsefalsefalse9falsefalsefalse00falsefalsefalsexbrli:monetaryItemTypemonetaryCarrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 false2Regulatory Assets, Current8false 0truefalsetruefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalse7falsefalsefalse00falsefalsefalse8falsefalsefalse00falsefalsefalse9falsefalsefalse00falsefalsefalse13false USDtruefalse$I2013Q2_us-gaap_RegulatoryAssetAxis_us-gaap_PensionAndOtherPostretirementPlansCostsMemberhttp://www.sec.gov/CIK0000073020instant2013-06-30T00:00:000001-01-01T00:00:00falsefalsePension and Other Postretirement Plans Costs [Member]us-gaap_RegulatoryAssetAxisxbrldihttp://xbrl.org/2006/xbrldius-gaap_PensionAndOtherPostretirementPlansCostsMemberus-gaap_RegulatoryAssetAxisexplicitMemberusdStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$nanafalse0Pension and Other Postretirement Plans Costs [Member]9true 3us-gaap_RegulatoryAssetLineItemsus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalse7falsefalsefalse00falsefalsefalse8falsefalsefalse00falsefalsefalse9falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringfalse0Regulatory Assets [Line Items]10false 4us-gaap_RegulatoryAssetsNoncurrentus-gaap_truedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse191312000191312[2]USD$falsefalsefalse2truefalsefalse182688000182688[2]USD$falsefalsefalse3falsefalsefalse00falsefalsefalse4truefalsefalse162767000162767[2]USD$falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalse7falsefalsefalse00falsefalsefalse8falsefalsefalse00falsefalsefalse9falsefalsefalse00falsefalsefalsexbrli:monetaryItemTypemonetaryCarrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 false2Regulatory Assets, Noncurrent11false 0truefalsetruefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalse7falsefalsefalse00falsefalsefalse8falsefalsefalse00falsefalsefalse9falsefalsefalse00falsefalsefalse16false USDtruefalse$I2013Q2_us-gaap_RegulatoryAssetAxis_us-gaap_PensionCostsMemberhttp://www.sec.gov/CIK0000073020instant2013-06-30T00:00:000001-01-01T00:00:00falsefalsePension Balancing [Member]us-gaap_RegulatoryAssetAxisxbrldihttp://xbrl.org/2006/xbrldius-gaap_PensionCostsMemberus-gaap_RegulatoryAssetAxisexplicitMemberusdStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$nanafalse0Pension Balancing [Member]12true 3us-gaap_RegulatoryAssetLineItemsus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalse7falsefalsefalse00falsefalsefalse8falsefalsefalse00falsefalsefalse9falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringfalse0Regulatory Assets [Line Items]13false 4us-gaap_RegulatoryAssetsNoncurrentus-gaap_truedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse2032700020327[2]USD$falsefalsefalse2truefalsefalse1472700014727[2]USD$falsefalsefalse3falsefalsefalse00falsefalsefalse4truefalsefalse1061100010611[2]USD$falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalse7falsefalsefalse00falsefalsefalse8falsefalsefalse00falsefalsefalse9falsefalsefalse00falsefalsefalsexbrli:monetaryItemTypemonetaryCarrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 false2Regulatory Assets, Noncurrent14false 0truefalsetruefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalse7falsefalsefalse00falsefalsefalse8falsefalsefalse00falsefalsefalse9falsefalsefalse00falsefalsefalse19false USDtruefalse$I2013Q2_us-gaap_RegulatoryAssetAxis_us-gaap_DeferredIncomeTaxChargesMemberhttp://www.sec.gov/CIK0000073020instant2013-06-30T00:00:000001-01-01T00:00:00falsefalseIncome Tax Asset [Member]us-gaap_RegulatoryAssetAxisxbrldihttp://xbrl.org/2006/xbrldius-gaap_DeferredIncomeTaxChargesMemberus-gaap_RegulatoryAssetAxisexplicitMemberusdStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$nanafalse0Income Tax Asset [Member]15true 3us-gaap_RegulatoryAssetLineItemsus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalse7falsefalsefalse00falsefalsefalse8falsefalsefalse00falsefalsefalse9falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringfalse0Regulatory Assets [Line Items]16false 4us-gaap_RegulatoryAssetsNoncurrentus-gaap_truedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse5306500053065USD$falsefalsefalse2truefalsefalse5587900055879USD$falsefalsefalse3falsefalsefalse00falsefalsefalse4truefalsefalse6345200063452USD$falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalse7falsefalsefalse00falsefalsefalse8falsefalsefalse00falsefalsefalse9falsefalsefalse00falsefalsefalsexbrli:monetaryItemTypemonetaryCarrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 false2Regulatory Assets, Noncurrent17false 0truefalsetruefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalse7falsefalsefalse00falsefalsefalse8falsefalsefalse00falsefalsefalse9falsefalsefalse00falsefalsefalse22false USDtruefalse$I2013Q2_us-gaap_RegulatoryAssetAxis_us-gaap_EnvironmentalRestorationCostsMemberhttp://www.sec.gov/CIK0000073020instant2013-06-30T00:00:000001-01-01T00:00:00falsefalseEnvironmental Restoration Costs [Member]us-gaap_RegulatoryAssetAxisxbrldihttp://xbrl.org/2006/xbrldius-gaap_EnvironmentalRestorationCostsMemberus-gaap_RegulatoryAssetAxisexplicitMemberusdStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$nanafalse0Environmental Restoration Costs [Member]18true 3us-gaap_RegulatoryAssetLineItemsus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalse7falsefalsefalse00falsefalsefalse8falsefalsefalse00falsefalsefalse9falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringfalse0Regulatory Assets [Line Items]19false 4us-gaap_RegulatoryAssetsNoncurrentus-gaap_truedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse120224000120224[3],[4]USD$falsefalsefalse2truefalsefalse121144000121144[3],[4]USD$falsefalsefalse3falsefalsefalse00falsefalsefalse4truefalsefalse113369000113369[3],[4]USD$falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalse7falsefalsefalse00falsefalsefalse8falsefalsefalse00falsefalsefalse9falsefalsefalse00falsefalsefalsexbrli:monetaryItemTypemonetaryCarrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 false2Regulatory Assets, Noncurrent20false 0truefalsetruefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalse7falsefalsefalse00falsefalsefalse8falsefalsefalse00falsefalsefalse9falsefalsefalse00falsefalsefalse25false USDtruefalse$I2013Q2_us-gaap_RegulatoryAssetAxis_us-gaap_OtherRegulatoryAssetsLiabilitiesMemberhttp://www.sec.gov/CIK0000073020instant2013-06-30T00:00:000001-01-01T00:00:00falsefalseOther Regulatory [Member]us-gaap_RegulatoryAssetAxisxbrldihttp://xbrl.org/2006/xbrldius-gaap_OtherRegulatoryAssetsLiabilitiesMemberus-gaap_RegulatoryAssetAxisexplicitMemberusdStandardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$nanafalse0Other Regulatory [Member]21true 3us-gaap_RegulatoryAssetLineItemsus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalseterseLabel1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalse3falsefalsefalse00falsefalsefalse4falsefalsefalse00falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalse7falsefalsefalse00falsefalsefalse8falsefalsefalse00falsefalsefalse9falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringfalse0Regulatory Assets [Line Items]22false 4us-gaap_RegulatoryAssetsNoncurrentus-gaap_truedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse69700006970[5]USD$falsefalsefalse2truefalsefalse72390007239[5]USD$falsefalsefalse3falsefalsefalse00falsefalsefalse4truefalsefalse99610009961[5]USD$falsefalsefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalse7falsefalsefalse00falsefalsefalse8falsefalsefalse00falsefalsefalse9falsefalsefalse00falsefalsefalsexbrli:monetaryItemTypemonetaryCarrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 false2Regulatory Assets, Noncurrent23false 4us-gaap_RegulatoryAssetsCurrentus-gaap_truedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse1656000016560[5]USD$falsetruefalse2truefalsefalse4165200041652[5]USD$falsetruefalse3falsefalsefalse00falsefalsefalse4truefalsefalse3589000035890[5]USD$falsetruefalse5falsefalsefalse00falsefalsefalse6falsefalsefalse00falsefalsefalse7falsefalsefalse00falsefalsefalse8falsefalsefalse00falsefalsefalse9falsefalsefalse00falsefalsefalsexbrli:monetaryItemTypemonetaryCarrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 false2Regulatory Assets, Current1Unrealized gains or losses on derivatives are non-cash items and, therefore, do not earn a rate of return or a carrying charge. These amounts are recoverable through utility rates as part of the annual Purchased Gas Adjustment (PGA) mechanism when realized at settlement.2Certain utility pension costs are approved for regulatory deferral, including amounts recorded to the pension balancing account, to mitigate the effects of higher and lower pension expenses. Pension costs that are deferred include an interest component when recognized in net periodic benefit costs. See Note 7.3Environmental costs relate to specific sites approved for regulatory deferral by the Public Utility Commission of Oregon (OPUC) and Washington Utilities and Transportation Commission (WUTC). In Oregon, we earn a carrying charge on amounts paid, whereas amounts accrued but not yet paid do not earn a carrying charge until expended. In Washington, a carrying charge related to deferred amounts will be determined in a future proceeding. In the 2012 Oregon general rate case, the OPUC authorized a Site Remediation and Recovery Mechanism (SRRM) that allows the Company to recover prudently incurred environmental costs, subject to an earnings test. For further information on environmental matters, see Note 13 and Note 15.4Total regulatory asset deferral includes cash paid, remaining liability, and interest, net of insurance reimbursement.5Other primarily consists of several deferrals and amortizations under other approved regulatory mechanisms. The accounts being amortized typically earn a rate of return or carrying charge.falseSummary of Significant Accounting Policies Regulatory Assets Disclosure (Details) (USD $)ThousandsUnKnownUnKnownUnKnowntruefalsefalseSheethttp://www.nwnatural.com/role/SummaryOfSignificantAccountingPoliciesRegulatoryAssetsDisclosureDetails923