2.2.0.25falsefalse10700 - Disclosure - Cost and Fair Value Basis of Long-Term Debttruefalsefalse1falsefalseUSDfalsefalse1/1/2011 - 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_DisclosureCostAndFairValueBasisOfLongTermDebtnwnfalsenadurationNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalsefalsefalseOtherxbrli:stringItemTypestringNo definition available.falsefalseDisclosure - Cost and Fair Value Basis of Long-Term Debt3false0us-gaap_LongTermDebtTextBlockus-gaaptruenadurationNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse007.       Cost and Fair Value Basis of Long-Term Debt Cost of Long-Term Debt Our long-term debt consists of medium-term notes (MTNs) with maturity dates from 2011 through 2035, interest rates ranging from 3.95 percent to 9.05 percent, and a weighted-average coupon rate of 6.17 percent. For the three months ended March 31, 2011, we did not issue or redeem any MTNs. For more detail on our outstanding long-term debt, see Note 7 in our 2010 Form 10-K. Fair Value of Long-Term Debt The following table provides an estimate of the fair value of our long-term debt, including current maturities of long-term debt, using market prices in effect on the valuation date.  Because our debt outstanding does not trade in active markets, we used interest rates for other company's outstanding debt issues that actively trade and have similar credit ratings, terms and remaining maturities to estimate fair value of our long-term debt issues. These are significant other observable inputs, or level 2 inputs, in the fair value hierarchy. <div><table style="border-collapse:collapse;margin-top:20px;"><tr style="height: 20px"><td style="width: 360px; text-align:left;border-color:#000000;min-width:360px;"> </td><td style="width: 10px; text-align:left;border-color:#000000;min-width:10px;"> </td><td colspan="5" style="width: 230px; border-bottom-style:solid;border-bottom-width:1px;text-align:center;border-color:#000000;min-width:230px;">March 31,</td><td style="width: 10px; text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 10px; border-bottom-style:solid;border-bottom-width:1px;text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 100px; border-bottom-style:solid;border-bottom-width:1px;text-align:center;border-color:#000000;min-width:100px;">December 31,</td></tr><tr style="height: 20px"><td style="width: 360px; text-align:left;border-color:#000000;min-width:360px;">Thousands</td><td style="width: 10px; text-align:center;border-color:#000000;min-width:10px;"> </td><td style="width: 10px; border-top-style:solid;border-top-width:1px;border-bottom-style:solid;border-bottom-width:1px;text-align:center;border-color:#000000;min-width:10px;"> </td><td style="width: 100px; border-top-style:solid;border-top-width:1px;border-bottom-style:solid;border-bottom-width:1px;text-align:center;border-color:#000000;min-width:100px;">2011</td><td style="width: 10px; border-top-style:solid;border-top-width:1px;text-align:center;border-color:#000000;min-width:10px;"> </td><td style="width: 10px; border-top-style:solid;border-top-width:1px;border-bottom-style:solid;border-bottom-width:1px;text-align:center;border-color:#000000;min-width:10px;"> </td><td style="width: 100px; border-top-style:solid;border-top-width:1px;border-bottom-style:solid;border-bottom-width:1px;text-align:center;border-color:#000000;min-width:100px;">2010</td><td style="width: 10px; text-align:center;border-color:#000000;min-width:10px;"> </td><td style="width: 10px; border-top-style:solid;border-top-width:1px;border-bottom-style:solid;border-bottom-width:1px;text-align:center;border-color:#000000;min-width:10px;"> </td><td style="width: 100px; border-top-style:solid;border-top-width:1px;border-bottom-style:solid;border-bottom-width:1px;text-align:center;border-color:#000000;min-width:100px;">2010</td></tr><tr style="height: 20px"><td style="width: 360px; text-align:left;border-color:#000000;min-width:360px;">Carrying amount</td><td style="width: 10px; text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 10px; border-top-style:solid;border-top-width:1px;text-align:right;border-color:#000000;min-width:10px;">$</td><td style="width: 100px; border-top-style:solid;border-top-width:1px;text-align:right;border-color:#000000;min-width:100px;"> 601,700</td><td style="width: 10px; text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 10px; border-top-style:solid;border-top-width:1px;text-align:right;border-color:#000000;min-width:10px;">$</td><td style="width: 100px; border-top-style:solid;border-top-width:1px;text-align:right;border-color:#000000;min-width:100px;"> 636,700</td><td style="width: 10px; text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 10px; border-top-style:solid;border-top-width:1px;text-align:right;border-color:#000000;min-width:10px;">$</td><td style="width: 100px; border-top-style:solid;border-top-width:1px;text-align:right;border-color:#000000;min-width:100px;"> 601,700</td></tr><tr style="height: 20px"><td style="width: 360px; text-align:left;border-color:#000000;min-width:360px;">Estimated fair value</td><td style="width: 10px; text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 10px; text-align:right;border-color:#000000;min-width:10px;">$</td><td style="width: 100px; text-align:right;border-color:#000000;min-width:100px;"> 680,436</td><td style="width: 10px; text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 10px; text-align:right;border-color:#000000;min-width:10px;">$</td><td style="width: 100px; text-align:right;border-color:#000000;min-width:100px;"> 687,937</td><td style="width: 10px; text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 10px; text-align:right;border-color:#000000;min-width:10px;">$</td><td style="width: 100px; text-align:right;border-color:#000000;min-width:100px;"> 690,126</td></tr></table></div>7.       Cost and Fair Value Basis of Long-Term Debt Cost of Long-Term Debt Our long-term debt consists offalsefalsefalsefalsefalseOtherus-types:textBlockItemTypestringThis element may be used as a single block of text to encapsulate the entire disclosure for long-term borrowings including data and tables.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 falsefalseCost and Fair Value Basis of Long-Term Debt Text Block12Cost and Fair Value Basis of Long-Term DebtUnKnownUnKnownUnKnownUnKnownfalsetrue