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Data Type:	xbrli:monetaryItemType
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Sale of Stock, Commissions and Fees Paid

No definition available.

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Sale of Stock, Consideration Received

No definition available.

Name:	nwn_SaleOfStockConsiderationReceived
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Sale of Stock, Cumulative Commissions and Fees Paid Under the Program.

No definition available.

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Data Type:	xbrli:monetaryItemType
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The cumulative number of shares issued or sold under the program.

No definition available.

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Data Type:	xbrli:sharesItemType
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_ClassOfStockLineItems
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Face amount per share of no-par value common stock.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

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Total number of common shares of an entity that have been sold or granted to shareholders (includes common shares that were issued, repurchased and remain in the treasury). These shares represent capital invested by the firm's shareholders and owners, and may be all or only a portion of the number of shares authorized. Shares issued include shares outstanding and shares held in the treasury.