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Commitments and Contingencies (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Leases [Abstract]      
Land Buildings And Equipment Expiration Date through 2108    
Operating Leases, Rent Expense, Minimum Rentals $ 5,900,000.0us-gaap_OperatingLeasesRentExpenseMinimumRentals $ 5,100,000.0us-gaap_OperatingLeasesRentExpenseMinimumRentals $ 4,800,000.0us-gaap_OperatingLeasesRentExpenseMinimumRentals
Operating And Capital Leases Table [Abstract]      
Operating Leases, Future Minimum Payments Due, Current 5,487,000us-gaap_OperatingLeasesFutureMinimumPaymentsDueCurrent    
Operating Leases, Future Minimum Payments, Due in Two Years 5,457,000us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears    
Operating Leases, Future Minimum Payments, Due in Three Years 5,426,000us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears    
Operating Leases, Future Minimum Payments, Due in Four Years 5,301,000us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears    
Operating Leases, Future Minimum Payments, Due in Five Years 5,209,000us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears    
Operating Leases, Future Minimum Payments, Due Thereafter 29,802,000us-gaap_OperatingLeasesFutureMinimumPaymentsDueThereafter    
Operating Leases, Future Minimum Payments Due 56,682,000us-gaap_OperatingLeasesFutureMinimumPaymentsDue    
Capital Leases, Future Minimum Payments Due, Current 680,000us-gaap_CapitalLeasesFutureMinimumPaymentsDueCurrent    
Capital Leases, Future Minimum Payments Due in Two Years 564,000us-gaap_CapitalLeasesFutureMinimumPaymentsDueInTwoYears    
Capital Leases, Future Minimum Payments Due in Three Years 157,000us-gaap_CapitalLeasesFutureMinimumPaymentsDueInThreeYears    
Capital Leases, Future Minimum Payments Due in Four Years 3,000us-gaap_CapitalLeasesFutureMinimumPaymentsDueInFourYears    
Capital Leases, Future Minimum Payments Due in Five Years 0us-gaap_CapitalLeasesFutureMinimumPaymentsDueInFiveYears    
Capital Leases, Future Minimum Payments Due Thereafter 0us-gaap_CapitalLeasesFutureMinimumPaymentsDueThereafter    
Capital Leases, Future Minimum Payments Due 1,404,000us-gaap_CapitalLeasesFutureMinimumPaymentsDue    
Future Minimum Payments Due Current 6,167,000nwn_FutureMinimumPaymentsDueCurrent    
Future Minimum Payments Due In Two Years 6,021,000nwn_FutureMinimumPaymentsDueInTwoYears    
Future Minimum Payments Due In Three Years 5,583,000nwn_FutureMinimumPaymentsDueInThreeYears    
Future Minimum Payments Due In Four Years 5,304,000nwn_FutureMinimumPaymentsDueInFourYears    
Future Minimum Payments Due In Five Years 5,209,000nwn_FutureMinimumPaymentsDueInFiveYears    
Future Minimum Payments Due Thereafter 29,802,000nwn_FutureMinimumPaymentsDueThereafter    
Future Minimum Payments Due Total 58,086,000nwn_FutureMinimumPaymentsDueTotal    
Unrecorded Unconditional Purchase Obligations [Abstract]      
Unrecorded Unconditional Purchase Obligation, Purchases 94,300,000us-gaap_UnrecordedUnconditionalPurchaseObligationPurchases 98,200,000us-gaap_UnrecordedUnconditionalPurchaseObligationPurchases 94,300,000us-gaap_UnrecordedUnconditionalPurchaseObligationPurchases
Unrecorded Unconditional Purchase Obligation Offsetting Sales 4,800,000nwn_UnrecordedUnconditionalPurchaseObligationOffsettingSales 4,500,000nwn_UnrecordedUnconditionalPurchaseObligationOffsettingSales 4,200,000nwn_UnrecordedUnconditionalPurchaseObligationOffsettingSales
Gas Purchase Agreements [Member]      
Unrecorded Unconditional Purchase Obligation [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 132,382,000us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnFirstAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_GasPurchaseAgreementsMember		    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 0us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnSecondAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_GasPurchaseAgreementsMember		    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 0us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnThirdAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_GasPurchaseAgreementsMember		    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 0us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnFourthAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_GasPurchaseAgreementsMember		    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 0us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnFifthAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_GasPurchaseAgreementsMember		    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 0us-gaap_UnrecordedUnconditionalPurchaseObligationDueAfterFiveYears
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_GasPurchaseAgreementsMember		    
Unrecorded Unconditional Purchase Obligation 132,382,000us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_GasPurchaseAgreementsMember		    
Unrecorded Unconditional Purchase Obligation Amount Representing Interest 93,000nwn_UnrecordedUnconditionalPurchaseObligationAmountRepresentingInterest
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_GasPurchaseAgreementsMember		    
Unrecorded Unconditional Purchase Obligation Total At Present Value 132,289,000nwn_UnrecordedUnconditionalPurchaseObligationTotalAtPresentValue
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_GasPurchaseAgreementsMember		    
Pipeline Capacity Purchase Agreements [Member]      
Unrecorded Unconditional Purchase Obligation [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 80,925,000us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnFirstAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_PipelineCapacityPurchaseAgreementsMember		    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 79,211,000us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnSecondAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_PipelineCapacityPurchaseAgreementsMember		    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 58,827,000us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnThirdAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_PipelineCapacityPurchaseAgreementsMember		    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 50,792,000us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnFourthAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_PipelineCapacityPurchaseAgreementsMember		    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 26,686,000us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnFifthAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_PipelineCapacityPurchaseAgreementsMember		    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 205,313,000us-gaap_UnrecordedUnconditionalPurchaseObligationDueAfterFiveYears
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_PipelineCapacityPurchaseAgreementsMember		    
Unrecorded Unconditional Purchase Obligation 501,754,000us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_PipelineCapacityPurchaseAgreementsMember		    
Unrecorded Unconditional Purchase Obligation Amount Representing Interest 76,748,000nwn_UnrecordedUnconditionalPurchaseObligationAmountRepresentingInterest
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_PipelineCapacityPurchaseAgreementsMember		    
Unrecorded Unconditional Purchase Obligation Total At Present Value 425,006,000nwn_UnrecordedUnconditionalPurchaseObligationTotalAtPresentValue
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_PipelineCapacityPurchaseAgreementsMember		    
Pipeline Capacity Release Agreements [Member]      
Unrecorded Unconditional Purchase Obligation [Line Items]      
Unrecorded Unconditional Purchase Obligation, Due within One Year 3,379,000us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnFirstAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_PipelineCapacityReleaseAgreementsMember		    
Unrecorded Unconditional Purchase Obligation, Due within Two Years 0us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnSecondAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_PipelineCapacityReleaseAgreementsMember		    
Unrecorded Unconditional Purchase Obligation, Due within Three Years 0us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnThirdAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_PipelineCapacityReleaseAgreementsMember		    
Unrecorded Unconditional Purchase Obligation, Due within Four Years 0us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnFourthAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_PipelineCapacityReleaseAgreementsMember		    
Unrecorded Unconditional Purchase Obligation, Due within Five Years 0us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceOnFifthAnniversary
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_PipelineCapacityReleaseAgreementsMember		    
Unrecorded Unconditional Purchase Obligation, Due after Five Years 0us-gaap_UnrecordedUnconditionalPurchaseObligationDueAfterFiveYears
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_PipelineCapacityReleaseAgreementsMember		    
Unrecorded Unconditional Purchase Obligation 3,379,000us-gaap_UnrecordedUnconditionalPurchaseObligationBalanceSheetAmount
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_PipelineCapacityReleaseAgreementsMember		    
Unrecorded Unconditional Purchase Obligation Amount Representing Interest 4,000nwn_UnrecordedUnconditionalPurchaseObligationAmountRepresentingInterest
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_PipelineCapacityReleaseAgreementsMember		    
Unrecorded Unconditional Purchase Obligation Total At Present Value $ 3,375,000nwn_UnrecordedUnconditionalPurchaseObligationTotalAtPresentValue
/ us-gaap_UnrecordedUnconditionalPurchaseObligationByCategoryOfItemPurchasedAxis
= nwn_PipelineCapacityReleaseAgreementsMember