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Gas Reserves (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Mar. 31, 2014
Gas Reserves [Abstract]      
Amended Gas Reserves Agreement Date Mar. 28, 2014    
Cumulative gross investment Jonah Wells $ 10,000nwn_CumulativegrossinvestmentJonahWells    
Gas Reserves, Current 19,112nwn_GasReservesCurrent 20,020nwn_GasReservesCurrent 21,990nwn_GasReservesCurrent
Gas Reserves Noncurrent Gross 168,352nwn_GasReservesNoncurrentGross 167,190nwn_GasReservesNoncurrentGross 156,450nwn_GasReservesNoncurrentGross
Gas Reserves Amortization 43,165nwn_GasReservesAmortization 37,910nwn_GasReservesAmortization 21,556nwn_GasReservesAmortization
Total Gas Reserves 144,299nwn_TotalGasReserves [1] 149,300nwn_TotalGasReserves [1] 156,884nwn_TotalGasReserves [1]
Deferred Taxes Related To Gas Reserves 28,383nwn_DeferredTaxesRelatedToGasReserves 18,551nwn_DeferredTaxesRelatedToGasReserves 30,704nwn_DeferredTaxesRelatedToGasReserves
Net investment in gas reserves 115,916nwn_NetGasReservesInvestment [1] 130,749nwn_NetGasReservesInvestment [1] 126,180nwn_NetGasReservesInvestment [1]
Total gas reserves investment Jonah Wells 9,200nwn_TotalgasreservesinvestmentJonahWells    
Net gas reserves investment Jonah Wells $ 8,300nwn_NetgasreservesinvestmentJonahWells    
[1] Total gas reserves includes our investment in additional wells, subject to regulatory deferral approvals, with total gas reserves of $9.2 million and net investment of $8.3 million at March 31, 2015 and no net investment or total gas reserves from additional wells at March 31, 2014.