2.2.0.25truefalse003000 - Statement - Consolidated Balance SheetstruefalseIn Millionsfalse1falsefalseUSDfalsefalse1/1/2010 - 12/31/2010 USD ($) USD ($) / shares $c00032http://www.sec.gov/CIK0000072020duration2010-01-01T00:00:002010-12-31T00:00:00u000Standardhttp://www.xbrl.org/2003/iso4217USDiso42170u001Standardhttp://www.xbrl.org/2003/instancesharesxbrli0u002Dividehttp://www.xbrl.org/2003/iso4217USDiso4217http://www.xbrl.org/2003/instancesharesxbrli0USDUSD$2falsefalseUSDfalsefalse1/1/2009 - 12/31/2009 USD ($) USD ($) / shares $c00005http://www.sec.gov/CIK0000072020duration2009-01-01T00:00:002009-12-31T00:00:00u000Standardhttp://www.xbrl.org/2003/iso4217USDiso42170u001Standardhttp://www.xbrl.org/2003/instancesharesxbrli0u002Dividehttp://www.xbrl.org/2003/iso4217USDiso4217http://www.xbrl.org/2003/instancesharesxbrli0USDUSD$2true0us-gaap_AssetsCurrentAbstractus-gaaptruenadurationNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalsefalsefalse2falsefalsefalse00falsefalsefalsefalsefalseOtherxbrli:stringItemTypestringNo definition available.falsefalseCurrent assets3false0us-gaap_CashAndCashEquivalentsAtCarryingValueus-gaaptruedebitinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse3150000031.5falsetruefalsefalsefalse2truefalsefalse5570000055.7falsetruefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryIncludes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Entity may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. Compensating balance arrangements that do not legally restrict the withdrawal or usage of cash amounts may be reported as Cash and Cash Equivalents, while legally restricted deposits held as compensating balances against borrowing arrangements, contracts entered into with others, or company statements of intention with regard to particular deposits should not be reported as cash and cash equivalents.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 7, 26 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 8, 9 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 95 -Paragraph 7 -Footnote 1 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 1 -Article 5 falsefalseCash and cash equivalents4false0us-gaap_ShortTermInvestmentsus-gaaptruedebitinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse7060000070.6falsefalsefalsefalsefalse2truefalsefalse7800000078.0falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryInvestments which are intended to be sold in the short term (usually less than one year or the normal operating cycle, whichever is longer) including trading securities, available-for-sale securities, held-to-maturity securities, and other short-term investments not otherwise listed in the existing taxonomy.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 1 -Subparagraph g -Article 7 falsefalseShort-term investments5false0us-gaap_AccountsReceivableNetCurrentus-gaaptruedebitinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalselabel1truefalsefalse472400000472.4falsefalsefalsefalsefalse2truefalsefalse492100000492.1falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryAmount due from customers or clients, within one year of the balance sheet date (or the normal operating cycle, whichever is longer), for goods or services (including trade receivables) that have been delivered or sold in the normal course of business, reduced to the estimated net realizable fair value by an allowance established by the entity of the amount it deems uncertain of collection.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 3 -Subparagraph a(1) -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 4 -Article 5 falsefalseReceivables, less allowances of $27.6 and $33.0, respectively6false0us-gaap_EnergyRelatedInventoryGasStoredUndergroundus-gaaptruedebitinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse163900000163.9falsefalsefalsefalsefalse2truefalsefalse137200000137.2falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryCarrying amount as of the balance sheet date of natural gas stored underground in depleted gas reservoirs, aquifers, or salt caverns to meet seasonal and peak load demands, and also as insurance against unforeseen supply disruptions, and deemed to be a current asset because it is expected to be used within twelve months or in the normal operating cycle.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 6 -Subparagraph a -Article 5 falsefalseGas in storage7false0us-gaap_DerivativeInstrumentsAndHedgesus-gaaptruedebitinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalseterselabel1truefalsefalse4910000049.1falsefalsefalsefalsefalse2truefalsefalse3090000030.9falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryCarrying amount as of the balance sheet date of the asset arising from derivative instruments and hedging activities, which are expected to be converted into cash or otherwise disposed of within a year or the normal operating cycle, if longer.No authoritative reference available.falsefalseDerivative instruments8false0us-gaap_MarginDepositAssetsus-gaaptruedebitinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalseterselabel1truefalsefalse5090000050.9falsefalsefalsefalsefalse2truefalsefalse4610000046.1falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryThe amount of cash or securities placed with a broker or counterparty as security for a trading or derivatives securities position which was partially obtained with funds provided by the broker dealer.No authoritative reference available.falsefalseMargin accounts - derivative instruments9false0us-gaap_OtherAssetsCurrentus-gaaptruedebitinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalseterselabel1truefalsefalse115200000115.2falsefalsefalsefalsefalse2truefalsefalse163300000163.3falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryAggregate carrying amount, as of the balance sheet date, of current assets not separately presented elsewhere in the balance sheet. Current assets are expected to be realized or consumed within one year (or the normal operating cycle, if longer).Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 8 -Article 5 falsefalseOther10false0us-gaap_AssetsCurrentus-gaaptruedebitinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalsetotallabel1truefalsefalse953600000953.6falsefalsefalsefalsefalse2truefalsefalse10033000001003.3falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetarySum of the carrying amounts as of the balance sheet date of all assets that are expected to be realized in cash, sold, or consumed within one year (or the normal operating cycle, if longer). Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 9 -Article 5 truefalseTotal current assets11true0us-gaap_PropertyPlantAndEquipmentGrossAbstractus-gaaptruenadurationNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalseterselabel1falsefalsefalse00falsefalsefalsefalsefalse2falsefalsefalse00falsefalsefalsefalsefalseOtherxbrli:stringItemTypestringNo definition available.falsefalseProperty, plant and equipment, at cost12false0us-gaap_PropertyPlantAndEquipmentGrossus-gaaptruedebitinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse51277000005127.7falsefalsefalsefalsefalse2truefalsefalse49610000004961.0falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryCarrying amount at the balance sheet date for long-lived physical assets used in the normal conduct of business and not intended for resale. This can include land, physical structures, machinery, vehicles, furniture, computer equipment, construction in progress, and similar items. Amount does not include depreciation.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 12 -Paragraph 5 falsefalseProperty, plant and equipment, at cost13false0us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipmentus-gaaptruecreditinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalselabel1truefalsefalse21049000002104.9falsefalsefalsefalsefalse2truefalsefalse20219000002021.9falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryThe cumulative amount of depreciation, depletion and amortization (related to property, plant and equipment, but not including land) that has been recognized in the income statement.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 12 -Paragraph 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 12 -Paragraph 5 -Subparagraph c Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 14 -Article 5 falsefalseLess accumulated depreciation14false0us-gaap_PropertyPlantAndEquipmentNetus-gaaptruedebitinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalsetotallabel1truefalsefalse30228000003022.8falsefalsefalsefalsefalse2truefalsefalse29391000002939.1falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryTangible assets that are held by an entity for use in the production or supply of goods and services, for rental to others, or for administrative purposes and that are expected to provide economic benefit for more than one year; net of accumulated depreciation. Examples include land, buildings, and production equipment.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 13 -Subparagraph a -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 12 -Paragraph 5 -Subparagraph b, c Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 8 -Article 7 truefalseTotal property, plant and equipment, net15false0us-gaap_LongTermInvestmentsus-gaaptruedebitinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse134200000134.2falsefalsefalsefalsefalse2truefalsefalse128800000128.8falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryThe total amount of investments that are intended to be held for an extended period of time (longer than one operating cycle).No authoritative reference available.falsefalseLong-term investments16false0us-gaap_OtherAssetsNoncurrentus-gaaptruedebitinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalseterselabel1truefalsefalse385900000385.9falsefalsefalsefalsefalse2truefalsefalse364500000364.5falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryAggregate carrying amount, as of the balance sheet date, of noncurrent assets not separately disclosed in the balance sheet due to materiality considerations. Noncurrent assets are expected to be realized or consumed after one year (or the normal operating cycle, if longer).Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 17 -Article 5 falsefalseOther assets17false0us-gaap_Assetsus-gaaptruedebitinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalsetotallabel1truefalsefalse44965000004496.5falsefalsefalsefalsefalse2truefalsefalse44357000004435.7falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetarySum of the carrying amounts as of the balance sheet date of all assets that are recognized. Assets are probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Concepts (CON) -Number 6 -Paragraph 25 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 18 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 12 -Article 7 truefalseTotal assets18true0us-gaap_LiabilitiesCurrentAbstractus-gaaptruenadurationNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalsefalsefalse2falsefalsefalse00falsefalsefalsefalsefalseOtherxbrli:stringItemTypestringNo definition available.falsefalseCurrent liabilities19false0us-gaap_CommercialPaperus-gaaptruecreditinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse425000000425.0falsefalsefalsefalsefalse2truefalsefalse494000000494.0falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryCarrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 16 -Article 7 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19 -Subparagraph a(3) -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 13 -Subparagraph 2 -Article 9 falsefalseShort-term debt20false0us-gaap_AccountsPayableAndAccruedLiabilitiesCurrentus-gaaptruecreditinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse335500000335.5falsefalsefalsefalsefalse2truefalsefalse353900000353.9falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryAccounts Payable and Accrued Liabilities, CurrentReference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 20 -Article 5 falsefalseAccounts payable21false0us-gaap_CustomerAdvancesAndDepositsCurrentus-gaaptruecreditinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse110600000110.6falsefalsefalsefalsefalse2truefalsefalse141700000141.7falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryThe current portion, due within one year or one operating cycle, if longer, of aggregate prepayments received from customers for goods or services to be provided in the future, as well as the current portion of money or property received from customers that are to be returned upon satisfactory contract completion or as partial prepayment for goods or services to be provided in the future.No authoritative reference available.falsefalseCustomer credit balances and deposits22false0us-gaap_HedgingLiabilitiesCurrentus-gaaptruecreditinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse8290000082.9falsefalsefalsefalsefalse2truefalsefalse7230000072.3falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryCarrying amount as of the balance sheet date of the liability arising from a financial instrument or a contract used to mitigate a specified risk (hedge), and which are expected to be converted into cash or otherwise disposed of within a year or the normal operating cycle, if longer.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 133 -Paragraph 17 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Audit and Accounting Guide (AAG) -Number AAG-BRD -Chapter 7 -Paragraph 41 -IssueDate 2006-05-01 falsefalseDerivative instruments23false0us-gaap_OtherLiabilitiesCurrentus-gaaptruecreditinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalseterselabel1truefalsefalse120300000120.3falsefalsefalsefalsefalse2truefalsefalse106200000106.2falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryAggregate carrying amount, as of the balance sheet date, of current obligations not separately disclosed in the balance sheet due to materiality considerations. Current liabilities are expected to be paid within one year (or the normal operating cycle, if longer).Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 20 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Research Bulletin (ARB) -Number 43 -Chapter 3 -Section A -Paragraph 8 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 6 -Paragraph 15 falsefalseOther24false0us-gaap_LiabilitiesCurrentus-gaaptruecreditinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalsetotallabel1truefalsefalse10743000001074.3falsefalsefalsefalsefalse2truefalsefalse11681000001168.1falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryTotal obligations incurred as part of normal operations that are expected to be paid during the following twelve months or within one business cycle, if longer.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 21 -Article 5 truefalseTotal current liabilities25true0us-gaap_LiabilitiesNoncurrentAbstractus-gaaptruenadurationNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalsefalsefalse2falsefalsefalse00falsefalsefalsefalsefalseOtherxbrli:stringItemTypestringNo definition available.falsefalseDeferred credits and other liabilities26false0gas_RegulatoryAssetRetirementCostLiabilityNoncurrentgasfalsecreditinstantRegulatory noncurrent liability related to non-legal obligations associated with the disposal or retirement of tangible...falsefalsefalsefalsefalsefalsefalsefalsefalsefalseterselabel1truefalsefalse843900000843.9falsefalsefalsefalsefalse2truefalsefalse796800000796.8falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryRegulatory noncurrent liability related to non-legal obligations associated with the disposal or retirement of tangible long-lived assets.No authoritative reference available.falsefalseRegulatory asset retirement liability27false0us-gaap_DeferredTaxLiabilitiesNoncurrentus-gaaptruecreditinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse423400000423.4falsefalsefalsefalsefalse2truefalsefalse409900000409.9falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryRepresents the noncurrent portion of deferred tax liabilities, which result from applying the applicable tax rate to net taxable temporary differences pertaining to each jurisdiction to which the entity is obligated to pay income tax. A noncurrent taxable temporary difference is a difference between the tax basis and the carrying amount of a noncurrent asset or liability in the financial statements prepared in accordance with generally accepted accounting principles. In a classified statement of financial position, an enterprise shall separate deferred tax liabilities and assets into a current amount and a noncurrent amount. Deferred tax liabilities and assets shall be classified as current or noncurrent based on the classification of the related asset or liability for financial reporting. A deferred tax liability or asset that is not related to an asset or liability for financial reporting, including deferred tax assets related to carryforwards, shall be classified according to the expected reversal date of the temporary difference.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 109 -Paragraph 41, 42 falsefalseDeferred income taxes28false0us-gaap_OtherPostretirementDefinedBenefitPlanLiabilitiesNoncurrentus-gaaptruecreditinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse229700000229.7falsefalsefalsefalsefalse2truefalsefalse199700000199.7falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryThis represents the noncurrent liability recognized in the balance sheet that is associated with other postretirement defined benefit plans (excluding pension plans). (The current liability will be separate, but it will normally be small, if there is even any at all.)Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph c Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 6 falsefalseHealth care and other postretirement benefits29false0us-gaap_AssetRetirementObligationsNoncurrentus-gaaptruecreditinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse190900000190.9falsefalsefalsefalsefalse2truefalsefalse191600000191.6falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryNoncurrent portion of the 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 obligation30false0us-gaap_OtherLiabilitiesNoncurrentus-gaaptruecreditinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalseterselabel1truefalsefalse132000000132.0falsefalsefalsefalsefalse2truefalsefalse133600000133.6falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryAggregate carrying amount, as of the balance sheet date, of noncurrent obligations not separately disclosed in the balance sheet due to materiality considerations. Noncurrent liabilities are expected to be paid after one year (or the normal operating cycle, if longer).Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 24 -Article 5 falsefalseOther31false0gas_TotalDeferredCreditsAndOtherLiabilitiesgasfalsecreditinstantTotal carrying amount as of the balance sheet date of unearned revenue or income which is expected to be taken into income...falsefalsefalsefalsefalsefalsefalsefalsefalsefalsetotallabel1truefalsefalse18199000001819.9falsefalsefalsefalsefalse2truefalsefalse17316000001731.6falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryTotal carrying amount as of the balance sheet date of unearned revenue or income which is expected to be taken into income after one year or beyond the normal operating cycle, if longer and total obligations incurred as part of normal operations that is expected to be repaid beyond the following twelve months or one business cycle.No authoritative reference available.truefalseTotal deferred credits and other liabilities32false0us-gaap_CommitmentsAndContingencies2009us-gaaptruenadurationNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00 falsefalsefalsefalsefalse2falsefalsefalse00 falsefalsefalsefalsefalseOtherxbrli:stringItemTypestringRepresents the caption on the face of the balance sheet to indicate that the entity has entered into (1) purchase or supply arrangements that will require expending a portion of its resources to meet the terms thereof, and (2) is exposed to potential losses or, less frequently, gains, arising from (a) possible claims against a company's resources due to future performance under contract terms, and (b) possible losses or likely gains from uncertainties that will ultimately be resolved when one or more future events that are deemed likely to occur do occur or fail to occur. This caption alerts the reader that one or more notes to the financial statements disclose pertinent information about the entity's commitments and contingencies.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 19 -Article 7 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 5 -Paragraph 8, 9 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 25 -Article 5 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 17 -Article 9 falsefalseCommitments and contingencies33true0us-gaap_CapitalizationLongtermDebtAndEquityAbstractus-gaaptruenadurationNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalsefalsefalse2falsefalsefalse00falsefalsefalsefalsefalseOtherxbrli:stringItemTypestringNo definition available.falsefalseCapitalization34false0gas_TotalLongTermObligationsgasfalsecreditinstantSum of the carrying values as of the balance sheet date of all long-term debt, which is debt initially having maturities due...falsefalsefalsefalsefalsefalsefalsefalsefalsefalseterselabel1truefalsefalse498400000498.4falsefalsefalsefalsefalse2truefalsefalse498300000498.3falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetarySum of the carrying values as of the balance sheet date of all long-term debt, which is debt initially having maturities due after one year from the balance sheet date or beyond the operating cycle, if longer, but excluding the portions thereof scheduled to be repaid within one year (current maturities) or the normal operating cycle, if longer, and after deducting unamortized discount or premiums, if any, comibned with the aggregate value of all preferred stock outstanding, that embody an unconditional obligation requiring the issuer to redeem the instrument by transferring its assets at a specified or determinable date (or dates) or upon an event certain to occur, including convertible preferred, excluding additional paid in capital, at the financial statement date.No authoritative reference available.falsefalseLong-term obligations35true0us-gaap_StockholdersEquityAbstractus-gaaptruenadurationNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1falsefalsefalse00falsefalsefalsefalsefalse2falsefalsefalse00falsefalsefalsefalsefalseOtherxbrli:stringItemTypestringNo definition available.falsefalseCommon equity36false0us-gaap_CommonStockValueus-gaaptruecreditinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse113900000113.9falsefalsefalsefalsefalse2truefalsefalse113100000113.1falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryDollar value of issued common stock whether issued at par value, no par or stated value. This item includes treasury stock repurchased by the entity. Note: elements for number of common shares, par value and other disclosure concepts are in another section within stockholders' equity.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 30 -Article 5 falsefalseCommon stock, $2.50 par value per share, (45,546,759 and 45,245,409 shares outstanding, respectively)37false0us-gaap_AdditionalPaidInCapitalus-gaaptruecreditinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse6910000069.1falsefalsefalsefalsefalse2truefalsefalse5460000054.6falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryExcess of issue price over par or stated value of the entity's capital stock and amounts received from other transactions involving the entity's stock or stockholders. Includes adjustments to additional paid in capital. Some examples of such adjustments include recording the issuance of debt with a beneficial conversion feature and certain tax consequences of equity instruments awarded to employees. Use this element for the aggregate amount of APIC associated with common AND preferred stock. For APIC associated with only common stock, use the element Additional Paid In Capital, Common Stock. For APIC associated with only preferred stock, use the element Additional Paid In Capital, Preferred Stock.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 31 -Article 5 falsefalsePaid-in capital38false0us-gaap_RetainedEarningsAccumulatedDeficitus-gaaptruecreditinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalseterselabel1truefalsefalse934100000934.1falsefalsefalsefalsefalse2truefalsefalse881000000881.0falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryThe cumulative amount of the reporting entity's undistributed earnings or deficit.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 12 -Paragraph 10 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 31 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 falsefalseRetained earnings39false0us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTaxus-gaaptruecreditinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalseterselabel1truefalsefalse-13200000-13.2falsefalsefalsefalsefalse2truefalsefalse-11000000-11.0falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryAccumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at fiscal year-end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, and unrealized gains and losses on certain investments in debt and equity securities as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Article 3 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 12 -Paragraph 10 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 130 -Paragraph 14, 17, 26 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 31 -Article 5 falsefalseAccumulated other comprehensive loss40false0us-gaap_StockholdersEquityus-gaaptruecreditinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalsetotallabel1truefalsefalse11039000001103.9falsefalsefalsefalsefalse2truefalsefalse10377000001037.7falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryTotal of all Stockholders' Equity (deficit) items, net of receivables from officers, directors owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Research Bulletin (ARB) -Number 51 -Paragraph A3 -Appendix A Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Staff Accounting Bulletin (SAB) -Number Topic 4 -Section E Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 29, 30, 31 -Article 5 truefalseTotal common equity41false0us-gaap_CapitalizationLongtermDebtAndEquityus-gaaptruecreditinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalsetotallabel1truefalsefalse16023000001602.3falsefalsefalsefalsefalse2truefalsefalse15360000001536.0falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryThis element represents the total consolidated (as applicable) capitalization of the entity which is comprised of its long-term debt and equity instruments. The table may be detailed by subsidiary (legal entity) and include information by type of debt or equity detailed by instrument.No authoritative reference available.truefalseTotal capitalization42false0us-gaap_LiabilitiesAndStockholdersEquityus-gaaptruecreditinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalsetotallabel1truefalsefalse44965000004496.5falsefalsefalsefalsefalse2truefalsefalse44357000004435.7falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryTotal of all Liabilities and Stockholders' Equity items.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 32 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 25 -Article 7 truefalseTotal liabilities and capitalization43false0natruenanaNo definition available.falsetruefalsefalsefalsefalsefalsefalsefalsefalsehttp://nicor.com/role/condensedconsolidatedbalancesheets1falsefalsefalse00falsefalsefalsefalsefalse2falsefalsefalse00falsefalsefalsefalsefalse5falsefalseUSDtruefalse{us-gaap_PropertyPlantAndEquipmentByTypeAxis} : Gas Distributions Equipment [Member] 12/31/2010 USD ($) $c00025http://www.sec.gov/CIK0000072020instant2010-12-31T00:00:000001-01-01T00:00:00falsefalseus-gaap_GasDistributionEquipmentMemberus-gaap_PropertyPlantAndEquipmentByTypeAxisxbrldihttp://xbrl.org/2006/xbrldius-gaap_GasDistributionEquipmentMemberus-gaap_PropertyPlantAndEquipmentByTypeAxisexplicitMemberu000Standardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$6falsefalseUSDtruefalse{us-gaap_PropertyPlantAndEquipmentByTypeAxis} : Gas Distributions Equipment [Member] 12/31/2009 USD ($) $c00016http://www.sec.gov/CIK0000072020instant2009-12-31T00:00:000001-01-01T00:00:00falsefalseus-gaap_GasDistributionEquipmentMemberus-gaap_PropertyPlantAndEquipmentByTypeAxisxbrldihttp://xbrl.org/2006/xbrldius-gaap_GasDistributionEquipmentMemberus-gaap_PropertyPlantAndEquipmentByTypeAxisexplicitMemberu000Standardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$OthernaNo definition available.No authoritative reference available.falsefalseGas Distributions Equipment [Member]45true0us-gaap_PropertyPlantAndEquipmentGrossAbstractus-gaaptruenadurationNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalseterselabel1falsefalsefalse00falsefalsefalsefalsefalse2falsefalsefalse00falsefalsefalsefalsefalseOtherxbrli:stringItemTypestringNo definition available.falsefalseProperty, plant and equipment, at cost46false0us-gaap_PropertyPlantAndEquipmentGrossus-gaaptruedebitinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse47336000004733.6falsefalsefalsefalsefalse2truefalsefalse45982000004598.2falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryCarrying amount at the balance sheet date for long-lived physical assets used in the normal conduct of business and not intended for resale. This can include land, physical structures, machinery, vehicles, furniture, computer equipment, construction in progress, and similar items. Amount does not include depreciation.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 12 -Paragraph 5 falsefalseProperty, plant and equipment, at cost51false0natruenanaNo definition available.falsetruefalsefalsefalsefalsefalsefalsefalsefalsehttp://nicor.com/role/condensedconsolidatedbalancesheets1falsefalsefalse00falsefalsefalsefalsefalse2falsefalsefalse00falsefalsefalsefalsefalse7falsefalseUSDtruefalse{us-gaap_PropertyPlantAndEquipmentByTypeAxis} : Shipping [Member] 12/31/2010 USD ($) $c00026http://www.sec.gov/CIK0000072020instant2010-12-31T00:00:000001-01-01T00:00:00falsefalsegas_ShippingMemberus-gaap_PropertyPlantAndEquipmentByTypeAxisxbrldihttp://xbrl.org/2006/xbrldigas_ShippingMemberus-gaap_PropertyPlantAndEquipmentByTypeAxisexplicitMemberu000Standardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$8falsefalseUSDtruefalse{us-gaap_PropertyPlantAndEquipmentByTypeAxis} : Shipping [Member] 12/31/2009 USD ($) $c00017http://www.sec.gov/CIK0000072020instant2009-12-31T00:00:000001-01-01T00:00:00falsefalsegas_ShippingMemberus-gaap_PropertyPlantAndEquipmentByTypeAxisxbrldihttp://xbrl.org/2006/xbrldigas_ShippingMemberus-gaap_PropertyPlantAndEquipmentByTypeAxisexplicitMemberu000Standardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$OthernaNo definition available.No authoritative reference available.falsefalseShipping [Member]53true0us-gaap_PropertyPlantAndEquipmentGrossAbstractus-gaaptruenadurationNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalseterselabel1falsefalsefalse00falsefalsefalsefalsefalse2falsefalsefalse00falsefalsefalsefalsefalseOtherxbrli:stringItemTypestringNo definition available.falsefalseProperty, plant and equipment, at cost54false0us-gaap_PropertyPlantAndEquipmentGrossus-gaaptruedebitinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse333600000333.6falsefalsefalsefalsefalse2truefalsefalse330000000330.0falsefalsefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryCarrying amount at the balance sheet date for long-lived physical assets used in the normal conduct of business and not intended for resale. This can include land, physical structures, machinery, vehicles, furniture, computer equipment, construction in progress, and similar items. Amount does not include depreciation.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 12 -Paragraph 5 falsefalseProperty, plant and equipment, at cost59false0natruenanaNo definition available.falsetruefalsefalsefalsefalsefalsefalsefalsefalsehttp://nicor.com/role/condensedconsolidatedbalancesheets1falsefalsefalse00falsefalsefalsefalsefalse2falsefalsefalse00falsefalsefalsefalsefalse9falsefalseUSDtruefalse{us-gaap_PropertyPlantAndEquipmentByTypeAxis} : Other Property Plant and Equipment [Member] 12/31/2010 USD ($) $c00027http://www.sec.gov/CIK0000072020instant2010-12-31T00:00:000001-01-01T00:00:00falsefalsegas_OtherPropertyPlantAndEquipmentMemberus-gaap_PropertyPlantAndEquipmentByTypeAxisxbrldihttp://xbrl.org/2006/xbrldigas_OtherPropertyPlantAndEquipmentMemberus-gaap_PropertyPlantAndEquipmentByTypeAxisexplicitMemberu000Standardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$10falsefalseUSDtruefalse{us-gaap_PropertyPlantAndEquipmentByTypeAxis} : Other Property Plant and Equipment [Member] 12/31/2009 USD ($) $c00018http://www.sec.gov/CIK0000072020instant2009-12-31T00:00:000001-01-01T00:00:00falsefalsegas_OtherPropertyPlantAndEquipmentMemberus-gaap_PropertyPlantAndEquipmentByTypeAxisxbrldihttp://xbrl.org/2006/xbrldigas_OtherPropertyPlantAndEquipmentMemberus-gaap_PropertyPlantAndEquipmentByTypeAxisexplicitMemberu000Standardhttp://www.xbrl.org/2003/iso4217USDiso42170USDUSD$OthernaNo definition available.No authoritative reference available.falsefalseOther Property Plant and Equipment [Member]61true0us-gaap_PropertyPlantAndEquipmentGrossAbstractus-gaaptruenadurationNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalseterselabel1falsefalsefalse00falsefalsefalsefalsefalse2falsefalsefalse00falsefalsefalsefalsefalseOtherxbrli:stringItemTypestringNo definition available.falsefalseProperty, plant and equipment, at cost62false0us-gaap_PropertyPlantAndEquipmentGrossus-gaaptruedebitinstantNo definition available.falsefalsefalsefalsefalsefalsefalsefalsefalsefalse1truefalsefalse6050000060.5falsetruefalsefalsefalse2truefalsefalse3280000032.8falsetruefalsefalsefalseMonetaryxbrli:monetaryItemTypemonetaryCarrying amount at the balance sheet date for long-lived physical assets used in the normal conduct of business and not intended for resale. This can include land, physical structures, machinery, vehicles, furniture, computer equipment, construction in progress, and similar items. Amount does not include depreciation.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher AICPA -Name Accounting Principles Board Opinion (APB) -Number 12 -Paragraph 5 falsefalseProperty, plant and equipment, at cost250Consolidated Balance Sheets (USD $)HundredThousandsUnKnownUnKnownUnKnownfalsetrue