2.2.0.25falsefalse11000 - Disclosure - Income Taxtruefalsefalse1falsefalseUSDfalsefalse1/1/2011 - 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_DisclosureIncomeTaxnwnfalsenadurationNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalsefalsefalseOtherxbrli:stringItemTypestringNo definition available.falsefalseDisclosure Income Tax3false0us-gaap_IncomeTaxDisclosureTextBlockus-gaaptruenadurationNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse0010.        Income Tax The effective income tax rate for the three months ended March 31, 2011 and 2010 varied from the combined federal and state statutory tax rates principally due to the following:<div><table style="border-collapse:collapse;margin-top:20px;"><tr style="height: 20px"><td style="width: 20px; text-align:left;border-color:#000000;min-width:20px;"> </td><td style="width: 510px; text-align:left;border-color:#000000;min-width:510px;"> </td><td style="width: 10px; border-bottom-style:solid;border-bottom-width:1px;text-align:left;border-color:#000000;min-width:10px;"> </td><td colspan="4" style="width: 180px; border-bottom-style:solid;border-bottom-width:1px;text-align:center;border-color:#000000;min-width:180px;">March 31,</td></tr><tr style="height: 20px"><td colspan="2" style="width: 530px; text-align:left;border-color:#000000;min-width:530px;"> </td><td style="width: 10px; border-top-style:solid;border-top-width:1px;border-bottom-style:solid;border-bottom-width:1px;text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 80px; border-top-style:solid;border-top-width:1px;border-bottom-style:solid;border-bottom-width:1px;text-align:center;border-color:#000000;min-width:80px;">2011</td><td style="width: 10px; border-top-style:solid;border-top-width:1px;border-bottom-style:solid;border-bottom-width:1px;text-align:left;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:left;border-color:#000000;min-width:10px;"> </td><td style="width: 80px; border-top-style:solid;border-top-width:1px;border-bottom-style:solid;border-bottom-width:1px;text-align:center;border-color:#000000;min-width:80px;">2010</td></tr><tr style="height: 20px"><td colspan="2" style="width: 530px; text-align:left;border-color:#000000;min-width:530px;">Federal statutory tax rate</td><td style="width: 10px; border-top-style:solid;border-top-width:1px;text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 80px; border-top-style:solid;border-top-width:1px;text-align:right;border-color:#000000;min-width:80px;">35.0%</td><td style="width: 10px; border-top-style:solid;border-top-width:1px;text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 10px; border-top-style:solid;border-top-width:1px;text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 80px; border-top-style:solid;border-top-width:1px;text-align:right;border-color:#000000;min-width:80px;">35.0%</td></tr><tr style="height: 20px"><td colspan="2" style="width: 530px; text-align:left;border-color:#000000;min-width:530px;">Increase (decrease):</td><td style="width: 10px; text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 80px; text-align:left;border-color:#000000;min-width:80px;"> </td><td style="width: 10px; text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 10px; text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 80px; text-align:left;border-color:#000000;min-width:80px;"> </td></tr><tr style="height: 20px"><td style="width: 20px; text-align:left;border-color:#000000;min-width:20px;"> </td><td style="width: 510px; text-align:left;border-color:#000000;min-width:510px;">Current state income tax, net of federal tax benefit</td><td style="width: 10px; text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 80px; text-align:right;border-color:#000000;min-width:80px;">4.6%</td><td style="width: 10px; text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 10px; text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 80px; text-align:right;border-color:#000000;min-width:80px;">4.9%</td></tr><tr style="height: 20px"><td style="width: 20px; text-align:left;border-color:#000000;min-width:20px;"> </td><td style="width: 510px; text-align:left;border-color:#000000;min-width:510px;">Amortization of investment and energy tax credits</td><td style="width: 10px; text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 80px; text-align:right;border-color:#000000;min-width:80px;">-0.4%</td><td style="width: 10px; text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 10px; text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 80px; text-align:right;border-color:#000000;min-width:80px;">-0.5%</td></tr><tr style="height: 20px"><td style="width: 20px; text-align:left;border-color:#000000;min-width:20px;"> </td><td style="width: 510px; text-align:left;border-color:#000000;min-width:510px;">Differences required to be flowed-through by regulatory commissions</td><td style="width: 10px; text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 80px; text-align:right;border-color:#000000;min-width:80px;">1.5%</td><td style="width: 10px; text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 10px; text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 80px; text-align:right;border-color:#000000;min-width:80px;">1.5%</td></tr><tr style="height: 20px"><td style="width: 20px; text-align:left;border-color:#000000;min-width:20px;"> </td><td style="width: 510px; text-align:left;border-color:#000000;min-width:510px;">Gains on company and trust-owned life insurance </td><td style="width: 10px; text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 80px; text-align:right;border-color:#000000;min-width:80px;">-0.2%</td><td style="width: 10px; text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 10px; text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 80px; text-align:right;border-color:#000000;min-width:80px;">-0.2%</td></tr><tr style="height: 20px"><td style="width: 20px; text-align:left;border-color:#000000;min-width:20px;"> </td><td style="width: 510px; text-align:left;border-color:#000000;min-width:510px;">Other - net</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: 80px; border-bottom-style:solid;border-bottom-width:1px;text-align:right;border-color:#000000;min-width:80px;">0.1%</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: 80px; border-bottom-style:solid;border-bottom-width:1px;text-align:right;border-color:#000000;min-width:80px;">0.1%</td></tr><tr style="height: 21px"><td colspan="2" style="width: 530px; text-align:left;border-color:#000000;min-width:530px;">Effective income tax rate</td><td style="width: 10px; border-top-style:solid;border-top-width:1px;border-bottom-style:double;border-bottom-width:3px;text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 80px; border-top-style:solid;border-top-width:1px;border-bottom-style:double;border-bottom-width:3px;text-align:right;border-color:#000000;min-width:80px;">40.6%</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;border-bottom-style:double;border-bottom-width:3px;text-align:left;border-color:#000000;min-width:10px;"> </td><td style="width: 80px; border-top-style:solid;border-top-width:1px;border-bottom-style:double;border-bottom-width:3px;text-align:right;border-color:#000000;min-width:80px;">40.8%</td></tr></table></div>The decrease in our effective tax rate for the three months ended March 31, 2011 compared to the same period in 2010 was minor and primarily due to a change in state income tax rates. See Note 10 in our 2010 Form 10-K.  10.        Income Tax The effective income tax rate for the three months ended March 31, 2011 and 2010 varied from thefalsefalsefalsefalsefalseOtherus-types:textBlockItemTypestringDescription containing the entire income tax disclosure. Examples include net deferred tax liability or asset recognized in an enterprise's statement of financial position, net change during the year in the total valuation allowance, approximate tax effect of each type of temporary difference and carryforward that gives rise to a significant portion of deferred tax liabilities and deferred tax assets, utilization of a tax carryback, and tax uncertainties information. This element may be used as a single block of text to encapsulate the entire disclosure including data and tables.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph h -Article 4 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 109 -Paragraph 136, 172 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 109 -Paragraph 43, 44, 45, 46, 47, 48, 49 falsefalseIncome Tax Text Block12Income TaxUnKnownUnKnownUnKnownUnKnownfalsetrue