2.2.0.25falsefalse11500 - Disclosure - Subsequent Eventtruefalsefalse1falsefalseUSDfalsefalse1/1/2011 - 3/31/2011 USD ($) USD ($) / shares $FROM_Jan01_2011_TO_Mar31_2011http://www.sec.gov/CIK0000073020duration2011-01-01T00:00:002011-03-31T00:00:00USDStandardhttp://www.xbrl.org/2003/iso4217USDiso42170SharesStandardhttp://www.xbrl.org/2003/instancesharesxbrli0USDpershareDividehttp://www.xbrl.org/2003/iso4217USDiso4217http://www.xbrl.org/2003/instancesharesxbrli0USDUSD$2true0nwn_DisclosureSubsequentEventsnwnfalsenadurationNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalsefalsefalseOtherxbrli:stringItemTypestringNo definition available.falsefalseDisclosure - Subsequent Events3false0us-gaap_ScheduleOfSubsequentEventsTextBlockus-gaaptruenadurationNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse0015.       Subsequent Event On April 28, 2011 the OPUC issued an order approving our investment to develop gas reserves on behalf of our Oregon customers under an agreement with Encana Oil & Gas (USA) Inc. Under the agreement, we expect to invest approximately $45-55 million a year, over a five-year period, for a total investment of about $250 million, which will cover a portion of drilling costs in exchange for working interests in certain sections of the Jonah Field located north of Rock Springs, Wyoming. These sections include both future and current producing wells. The gas reserves will provide long-term gas supplies for our Oregon utility customers over a period expected to be about 30 years. During the first 10 years of the agreement, we forecast to receive approximately 58 billion cubic feet (Bcf) from the transaction, or 8-10 percent of our average annual requirements for utility operations. Our total investment under the agreement is expected to result in about 93 Bcf of gas at an average all-in price of approximately $5.15 per dekatherm. We estimate net present value savings to customers of over $50 million over the life of the investment as compared to other long-term supply alternatives.  Under the order, the OPUC determined that the investment was prudent and that the Company is allowed to recover its costs under the agreement on an ongoing basis through its Purchased Gas Adjustment (PGA) cost sharing mechanism, including the deferral process for the commodity cost of gas. Annually, the Company will forecast amounts related to the costs and volumes expected, and variances will be subject to the PGA's normal sharing mechanism up to $10 million of variance. Any variance in excess of $10 million, either negative or positive, will be passed through to customers at 100 percent, rather than at the 80 or 90 percent level associated with the normal sharing mechanism. As part of the decision by the OPUC to approve the Company's investment, we will file a general rate case in Oregon no later than December 31, 2011. 15.       Subsequent Event On April 28, 2011 the OPUC issued an order approving our investment to develop gas reservesfalsefalsefalsefalsefalseOtherus-types:textBlockItemTypestringDescribes disclosed significant events or transactions that occurred after the balance sheet date, but before the issuance of the financial statements. Examples include: the sale of a capital stock issue, purchase of a business, settlement of litigation, losses resulting from fire or flood, losses on receivables, significant realized and unrealized gains and losses that result from changes in quoted market prices of securities, declines in market prices of inventory, changes in authorized or issued debt (SEC), significant foreign exchange rate changes, substantial loans to insiders or affiliates, significant long-term investments, and substantial dividends not in the ordinary course of business.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 5 -Paragraph 11 falsefalseSubsequent Events Text Block12Subsequent EventUnKnownUnKnownUnKnownUnKnownfalsetrue