2.2.0.25falsefalse009070 - Disclosure - Asset Retirement Obligations (Details)truefalseIn Millionsfalse1falsefalseUSDfalsefalse1/1/2011 - 3/31/2011 USD ($) USD ($) / shares $c20110101to20110331http://www.sec.gov/CIK0000072207duration2011-01-01T00:00:002011-03-31T00:00:00U003Standardhttp://www.xbrl.org/2003/instancesharesxbrli0U001Standardhttp://www.xbrl.org/2003/iso4217USDiso42170U002Dividehttp://www.xbrl.org/2003/iso4217USDiso4217http://www.xbrl.org/2003/instancesharesxbrli0U004Standardhttp://www.xbrl.org/2003/instancepurexbrli0USDUSD$2falsefalseUSDfalsefalse1/1/2010 - 3/31/2010 USD ($) USD ($) / shares $c20100101to20100331http://www.sec.gov/CIK0000072207duration2010-01-01T00:00:002010-03-31T00:00:00U001Standardhttp://www.xbrl.org/2003/iso4217USDiso42170U003Standardhttp://www.xbrl.org/2003/instancesharesxbrli0U002Dividehttp://www.xbrl.org/2003/iso4217USDiso4217http://www.xbrl.org/2003/instancesharesxbrli0U004Standardhttp://www.xbrl.org/2003/instancepurexbrli0USDUSD$2true0nbl_NotesToFinancialStatementsAbstractnblfalsenadurationNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalsefalsefalse2falsefalsefalse00falsefalsefalsefalsefalseOtherxbrli:stringItemTypestringNo definition available.falsefalseNotes to Financial Statements [Abstract]3false0us-gaap_AssetRetirementObligationus-gaaptruecreditinstantNo definition available.falsefalsefalsefalsefalsefalsefalsetruefalsefalseperiodstartlabel1truefalsefalse253000000253falsetruefalsefalsefalse2truefalsefalse232000000232falsetruefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryThe carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 143 -Paragraph 3, 10, 22 falsefalseAsset Retirement Obligations, Beginning Balance4false0us-gaap_AssetRetirementObligationLiabilitiesIncurredus-gaaptruecreditdurationNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalselabel1truefalsefalse10000001falsefalsefalsefalsefalse2truefalsefalse1400000014falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryAmount of asset retirement obligations incurred during the period.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 143 -Paragraph 22 -Subparagraph c(1) falsefalseLiabilities Incurred5false0us-gaap_AssetRetirementObligationLiabilitiesSettledus-gaaptruedebitdurationNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalselabel1truefalsefalse-9000000-9falsefalsefalsefalsefalse2truefalsefalse-4000000-4falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryAmount of asset retirement obligations settled, or otherwise disposed of, during the period. This may include asset retirement obligations transferred to third parties associated with the sale of a long-lived asset.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 143 -Paragraph 22 -Subparagraph c(2) falsefalseLiabilities Settled6false0us-gaap_AssetRetirementObligationRevisionOfEstimateus-gaaptruecreditdurationNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalselabel1truefalsefalse40000004falsefalsefalsefalsefalse2truefalsefalse40000004falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryAmount of the increase (decrease) in the amount of the asset retirement obligation during the current period for changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 143 -Paragraph 22 -Subparagraph c(4) falsefalseRevision of Estimate7false0us-gaap_AssetRetirementObligationAccretionExpenseus-gaaptruedebitdurationNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalseverboselabel1truefalsefalse50000005falsefalsefalsefalsefalse2truefalsefalse40000004falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryAmount of accretion expense recognized in the income statement during the period that is associated with asset retirement obligations. Accretion expense measures and incorporates changes due to the passage of time into the carrying amount of the liability. Such accretion expense representing changes in the liability for an asset retirement obligation due to passage of time is the result of applying an interest method of allocation to the amount of the liability at the beginning of the period. The interest rate used to measure that change shall be the credit-adjusted risk-free rate that existed when the liability, or portion thereof, was initially measured. That amount shall be recognized as an increase in the carrying amount of the liability and as an expense classified as an operating item in the statement of income. Accretion expense shall not be considered to be interest cost for purposes of capitalization of interest.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 143 -Paragraph 13, 14 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 143 -Paragraph 22 -Subparagraph c(3) falsefalseAccretion Expense8false0us-gaap_AssetRetirementObligationus-gaaptruecreditinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsetruefalseperiodendlabel1truefalsefalse254000000254falsetruefalsefalsefalse2truefalsefalse250000000250falsetruefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryThe carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 143 -Paragraph 3, 10, 22 falsefalseAsset Retirement Obligations, Ending Balance27Asset Retirement Obligations (Details) (USD $)MillionsUnKnownUnKnownUnKnownfalsetrue