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    <Log type="Warn">Presentation group ''Pension and Other Postretirement Benefits (Details)'', has multiple root nodes. XBRL allows unordered root nodes, but rendering requires ordering.  They will instead be ordered by their labels.  To avoid undesirable ordering of axes and primary items across multiple root nodes, rearrange the presentation group to have only a single root node.</Log>
    <Log type="Warn">Presentation group ''Derivative Instruments Gain (Loss) by Hedging Relationship, by Income Statement Location, by Derivative Instrument Risk (Details)'', has multiple root nodes. XBRL allows unordered root nodes, but rendering requires ordering.  They will instead be ordered by their labels.  To avoid undesirable ordering of axes and primary items across multiple root nodes, rearrange the presentation group to have only a single root node.</Log>
    <Log type="Warn">Presentation group ''Environmental Matters (Details)'', has multiple root nodes. XBRL allows unordered root nodes, but rendering requires ordering.  They will instead be ordered by their labels.  To avoid undesirable ordering of axes and primary items across multiple root nodes, rearrange the presentation group to have only a single root node.</Log>
    <Log type="Info">In ''Consolidated Balance Sheets'', column(s) 2, 5, 6 are contained in other reports, so were removed by flow through suppression.</Log>
    <Log type="Info">In ''Consolidated Statement of Cash Flows'', column(s) 1, 2 are contained in other reports, so were removed by flow through suppression.</Log>
  </Logs>
  <InputFiles>
    <File>nwn-20150930.xml</File>
    <File>nwn-20150930_cal.xml</File>
    <File>nwn-20150930_def.xml</File>
    <File>nwn-20150930_lab.xml</File>
    <File>nwn-20150930_pre.xml</File>
    <File>nwn-20150930.xsd</File>
  </InputFiles>
  <SupplementalFiles/>
  <BaseTaxonomies/>
  <HasPresentationLinkbase>true</HasPresentationLinkbase>
  <HasCalculationLinkbase>true</HasCalculationLinkbase>
</FilingSummary>

