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Rate and Regulatory Matters (Details) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Public Utilities, General Disclosures [Line Items]  
Annual cost of service $ 466.5nwp_AnnualCostOfService
General system firm transportation rate, per unit 0.44nwp_GeneralSystemFirmTransportationRatePerUnit
General system firm transportation rate, percentage 7.40%us-gaap_PublicUtilitiesApprovedRateIncreaseDecreasePercentage
Minimum [Member]  
Public Utilities, General Disclosures [Line Items]  
Duration of effective period of rates 3 years
Maximum [Member]  
Public Utilities, General Disclosures [Line Items]  
Duration of effective period of rates 5 years