2.2.0.7 true Condensed Consolidated Balance Sheets (Unaudited) (USD $) 003000 - Statement - Condensed Consolidated Balance Sheets (Unaudited) true false In Millions false false 1 USD false false u000 Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 u002 Divide http://www.xbrl.org/2003/iso4217 USD iso4217 http://www.xbrl.org/2003/instance shares xbrli 0 u001 Standard http://www.xbrl.org/2003/instance shares xbrli 0 $ false 2 USD false false u000 Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 u002 Divide http://www.xbrl.org/2003/iso4217 USD iso4217 http://www.xbrl.org/2003/instance shares xbrli 0 u001 Standard http://www.xbrl.org/2003/instance shares xbrli 0 $ false 3 USD false false u000 Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 $ 2 Current assets 2 us-gaap_AssetsCurrentAbstract us-gaap true na duration No definition available. false false false false false true false false false false false false 1 false false false false 0 0 false false false 2 false false false false 0 0 false false false 3 false false false false 0 0 false false false xbrli:stringItemType string No definition available. false 3 Cash and cash equivalents 2 us-gaap_CashAndCashEquivalentsAtCarryingValue us-gaap true debit instant No definition available. false false false false false false false false false false false false 1 true true false false 59800000 59.8 false false false 2 true true false false 43900000 43.9 false false false 3 true true false false 55700000 55.7 false false false xbrli:monetaryItemType monetary Includes 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 false 4 Short-term investments 2 us-gaap_ShortTermInvestments us-gaap true debit instant No definition available. false false false false false false false false false false false false 1 false true false false 66600000 66.6 false false false 2 false true false false 72400000 72.4 false false false 3 false true false false 78000000 78.0 false false false xbrli:monetaryItemType monetary Investments 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 false 5 Receivables, less allowances of $50.1, $33.0 and $51.9, respectively 2 us-gaap_AccountsReceivableNetCurrent us-gaap true debit instant No definition available. false false false false false false false false false false false label false 1 false true false false 268100000 268.1 false false false 2 false true false false 310900000 310.9 false false false 3 false true false false 492100000 492.1 false false false xbrli:monetaryItemType monetary Amount 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 false 6 Gas in storage 2 us-gaap_EnergyRelatedInventoryGasStoredUnderground us-gaap true debit instant No definition available. false false false false false false false false false false false false 1 false true false false 53400000 53.4 false false false 2 false true false false 28600000 28.6 false false false 3 false true false false 137200000 137.2 false false false xbrli:monetaryItemType monetary Carrying 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 false 7 Derivative instruments 2 us-gaap_DerivativeInstrumentsAndHedges us-gaap true debit instant No definition available. false false false false false false false false false false false terselabel false 1 false true false false 44400000 44.4 false false false 2 false true false false 53300000 53.3 false false false 3 false true false false 30900000 30.9 false false false xbrli:monetaryItemType monetary Carrying 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. false 8 Margin accounts - derivative instruments 2 us-gaap_MarginDepositAssets us-gaap true debit instant No definition available. false false false false false false false false false false false terselabel false 1 false true false false 52600000 52.6 false false false 2 false true false false 116800000 116.8 false false false 3 false true false false 46100000 46.1 false false false xbrli:monetaryItemType monetary The 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. false 9 Other 2 us-gaap_OtherAssetsCurrent us-gaap true debit instant No definition available. false false false false false false false false false false false terselabel false 1 false true false false 111900000 111.9 false false false 2 false true false false 117400000 117.4 false false false 3 false true false false 163300000 163.3 false false false xbrli:monetaryItemType monetary Aggregate 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 false 10 Total current assets 2 us-gaap_AssetsCurrent us-gaap true debit instant No definition available. false false false false false false false false false false false totallabel false 1 false true false false 656800000 656.8 false false false 2 false true false false 743300000 743.3 false false false 3 false true false false 1003300000 1003.3 false false false xbrli:monetaryItemType monetary Sum 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 true 11 Property, plant and equipment, at cost 2 us-gaap_PropertyPlantAndEquipmentGrossAbstract us-gaap true na duration No definition available. false false false false false true false false false false false terselabel false 1 false false false false 0 0 false false false 2 false false false false 0 0 false false false 3 false false false false 0 0 false false false xbrli:stringItemType string No definition available. false 12 Property, plant and equipment, at cost 2 us-gaap_PropertyPlantAndEquipmentGross us-gaap true debit instant No definition available. false false false false false false false false false false false false 1 false true false false 5029400000 5029.4 false false false 2 false true false false 4889300000 4889.3 false false false 3 false true false false 4961000000 4961.0 false false false xbrli:monetaryItemType monetary Carrying 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 false 13 Less accumulated depreciation 2 us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment us-gaap true credit instant No definition available. false false false false false false false false false false false label false 1 false true false false 2069400000 2069.4 false false false 2 false true false false 1990100000 1990.1 false false false 3 false true false false 2021900000 2021.9 false false false xbrli:monetaryItemType monetary The 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 false 14 Total property, plant and equipment, net 2 us-gaap_PropertyPlantAndEquipmentNet us-gaap true debit instant No definition available. false false false false false false false false false false false totallabel false 1 false true false false 2960000000 2960.0 false false false 2 false true false false 2899200000 2899.2 false false false 3 false true false false 2939100000 2939.1 false false false xbrli:monetaryItemType monetary Tangible 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 true 15 Long-term investments 2 us-gaap_LongTermInvestments us-gaap true debit instant No definition available. false false false false false false false false false false false false 1 false true false false 137000000 137.0 false false false 2 false true false false 128400000 128.4 false false false 3 false true false false 128800000 128.8 false false false xbrli:monetaryItemType monetary The 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. false 16 Other assets 2 us-gaap_OtherAssetsNoncurrent us-gaap true debit instant No definition available. false false false false false false false false false false false terselabel false 1 false true false false 373700000 373.7 false false false 2 false true false false 416700000 416.7 false false false 3 false true false false 364500000 364.5 false false false xbrli:monetaryItemType monetary Aggregate 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 false 17 Total assets 2 us-gaap_Assets us-gaap true debit instant No definition available. false false false false false false false false false false false totallabel false 1 false true false false 4127500000 4127.5 false false false 2 false true false false 4187600000 4187.6 false false false 3 false true false false 4435700000 4435.7 false false false xbrli:monetaryItemType monetary Sum 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 true 18 Current liabilities 2 us-gaap_LiabilitiesCurrentAbstract us-gaap true na duration No definition available. false false false false false true false false false false false false 1 false false false false 0 0 false false false 2 false false false false 0 0 false false false 3 false false false false 0 0 false false false xbrli:stringItemType string No definition available. false 19 Long-term debt due within one year 2 us-gaap_LongTermDebtCurrent us-gaap true credit instant No definition available. false false false false false false false false false false false false 1 false true false false 75000000 75.0 false false false 2 false true false false 0 0 false false false 3 false true false false 0 0 false false false xbrli:monetaryItemType monetary Total of the portions of the carrying amounts as of the balance sheet date of long-term debt, which may include notes payable, bonds payable, debentures, mortgage loans, and commercial paper, which are scheduled to be repaid within one year or the normal operating cycle, if longer, and after deducting unamortized discount or premiums, if any. 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 SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19 -Article 5 false 20 Short-term debt 2 us-gaap_CommercialPaper us-gaap true credit instant No definition available. false false false false false false false false false false false false 1 false true false false 107000000 107.0 false false false 2 false true false false 227000000 227.0 false false false 3 false true false false 494000000 494.0 false false false xbrli:monetaryItemType monetary Carrying 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 false 21 Accounts payable 2 us-gaap_AccountsPayableAndAccruedLiabilitiesCurrent us-gaap true credit instant No definition available. false false false false false false false false false false false false 1 false true false false 293900000 293.9 false false false 2 false true false false 266100000 266.1 false false false 3 false true false false 353900000 353.9 false false false xbrli:monetaryItemType monetary Accounts Payable and Accrued Liabilities, Current Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 20 -Article 5 false 22 Customer credit balances and deposits 2 us-gaap_CustomerAdvancesAndDepositsCurrent us-gaap true credit instant No definition available. false false false false false false false false false false false false 1 false true false false 101500000 101.5 false false false 2 false true false false 143100000 143.1 false false false 3 false true false false 141700000 141.7 false false false xbrli:monetaryItemType monetary The 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. false 23 Temporary LIFO inventory liquidation 2 us-gaap_ExcessOfReplacementOrCurrentCostsOverStatedLIFOValue us-gaap true debit instant No definition available. false false false false false false false false false false true negated false 1 false true false false 87700000 87.7 false false false 2 false true false false 82900000 82.9 false false false 3 false true false false 0 0 false false false xbrli:monetaryItemType monetary The excess of replacement or current cost over the stated LIFO (last in first out) inventory value when the LIFO inventory method is utilized. Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 6 -Subparagraph c -Article 5 false 24 Derivative instruments 2 us-gaap_HedgingLiabilitiesCurrent us-gaap true credit instant No definition available. false false false false false false false false false false false false 1 false true false false 88100000 88.1 false false false 2 false true false false 152400000 152.4 false false false 3 false true false false 72300000 72.3 false false false xbrli:monetaryItemType monetary Carrying 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 false 25 Other 2 us-gaap_OtherLiabilitiesCurrent us-gaap true credit instant No definition available. false false false false false false false false false false false terselabel false 1 false true false false 107800000 107.8 false false false 2 false true false false 104500000 104.5 false false false 3 false true false false 106200000 106.2 false false false xbrli:monetaryItemType monetary Aggregate 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 false 26 Total current liabilities 2 us-gaap_LiabilitiesCurrent us-gaap true credit instant No definition available. false false false false false false false false false false false totallabel false 1 false true false false 861000000 861.0 false false false 2 false true false false 976000000 976.0 false false false 3 false true false false 1168100000 1168.1 false false false xbrli:monetaryItemType monetary Total 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 true 27 Deferred credits and other liabilities 2 us-gaap_LiabilitiesNoncurrentAbstract us-gaap true na duration No definition available. false false false false false true false false false false false false 1 false false false false 0 0 false false false 2 false false false false 0 0 false false false 3 false false false false 0 0 false false false xbrli:stringItemType string No definition available. false 28 Regulatory asset retirement liability 2 gas_RegulatoryAssetRetirementCostLiabilityNoncurrent gas false credit instant Regulatory noncurrent liability related to non-legal obligations associated with the disposal or retirement of tangible... false false false false false false false false false false false terselabel false 1 false true false false 825700000 825.7 false false false 2 false true false false 774700000 774.7 false false false 3 false true false false 796800000 796.8 false false false xbrli:monetaryItemType monetary Regulatory noncurrent liability related to non-legal obligations associated with the disposal or retirement of tangible long-lived assets. No authoritative reference available. false 29 Deferred income taxes 2 us-gaap_DeferredTaxLiabilitiesNoncurrent us-gaap true credit instant No definition available. false false false false false false false false false false false false 1 false true false false 405300000 405.3 false false false 2 false true false false 399000000 399.0 false false false 3 false true false false 409900000 409.9 false false false xbrli:monetaryItemType monetary Represents 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 false 30 Health care and other postretirement benefits 2 us-gaap_OtherPostretirementDefinedBenefitPlanLiabilitiesNoncurrent us-gaap true credit instant No definition available. false false false false false false false false false false false false 1 false true false false 201000000 201.0 false false false 2 false true false false 197500000 197.5 false false false 3 false true false false 199700000 199.7 false false false xbrli:monetaryItemType monetary This 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 false 31 Asset retirement obligation 2 us-gaap_AssetRetirementObligationsNoncurrent us-gaap true credit instant No definition available. false false false false false false false false false false false false 1 false true false false 186500000 186.5 false false false 2 false true false false 188900000 188.9 false false false 3 false true false false 191600000 191.6 false false false xbrli:monetaryItemType monetary Noncurrent 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 false 32 Other 2 us-gaap_OtherLiabilitiesNoncurrent us-gaap true credit instant No definition available. false false false false false false false false false false false terselabel false 1 false true false false 136500000 136.5 false false false 2 false true false false 146900000 146.9 false false false 3 false true false false 133600000 133.6 false false false xbrli:monetaryItemType monetary Aggregate 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 false 33 Total deferred credits and other liabilities 2 gas_TotalDeferredCreditsAndOtherLiabilities gas false credit instant Total carrying amount as of the balance sheet date of unearned revenue or income which is expected to be taken into income... false false false false false false false false false false false totallabel false 1 false true false false 1755000000 1755.0 false false false 2 false true false false 1707000000 1707.0 false false false 3 false true false false 1731600000 1731.6 false false false xbrli:monetaryItemType monetary Total 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. true 34 Commitments and contingencies 2 us-gaap_CommitmentsAndContingencies2009 us-gaap true na duration No definition available. false false false false false false false false false false false false 1 false false false false 0 0   false false false 2 false false false false 0 0   false false false 3 false false false false 0 0   false false false xbrli:stringItemType string Represents 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 false 35 Long-term obligations 2 us-gaap_LongTermDebtNoncurrentAbstract us-gaap true na duration No definition available. false false false false false true false false false false false false 1 false false false false 0 0 false false false 2 false false false false 0 0 false false false 3 false false false false 0 0 false false false xbrli:stringItemType string No definition available. false 36 Long-term debt, net of unamortized discount 2 us-gaap_LongTermDebtNoncurrent us-gaap true credit instant No definition available. false false false false false false false false false false false false 1 false true false false 423300000 423.3 false false false 2 false true false false 498100000 498.1 false false false 3 false true false false 498200000 498.2 false false false xbrli:monetaryItemType monetary Sum 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. Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 22 -Article 5 false 37 Mandatorily redeemable preferred stock 2 us-gaap_RedeemablePreferredStock us-gaap true credit instant No definition available. false false false false false false false false false false false false 1 false true false false 100000 0.1 false false false 2 false true false false 600000 0.6 false false false 3 false true false false 100000 0.1 false false false xbrli:monetaryItemType monetary 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. Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 28 -Article 5 false 38 Total long-term obligations 2 gas_TotalLongTermObligations gas false credit instant Sum of the carrying values as of the balance sheet date of all long-term debt, which is debt initially having maturities due... false false false false false false false false false false false totallabel false 1 false true false false 423400000 423.4 false false false 2 false true false false 498700000 498.7 false false false 3 false true false false 498300000 498.3 false false false xbrli:monetaryItemType monetary Sum 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. true 39 Common equity 2 us-gaap_StockholdersEquityAbstract us-gaap true na duration No definition available. false false false false false true false false false false false false 1 false false false false 0 0 false false false 2 false false false false 0 0 false false false 3 false false false false 0 0 false false false xbrli:stringItemType string No definition available. false 40 Common stock 2 us-gaap_CommonStockValue us-gaap true credit instant No definition available. false false false false false false false false false false false false 1 false true false false 113800000 113.8 false false false 2 false true false false 113000000 113.0 false false false 3 false true false false 113100000 113.1 false false false xbrli:monetaryItemType monetary Dollar 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 false 41 Paid-in capital 2 us-gaap_AdditionalPaidInCapital us-gaap true credit instant No definition available. false false false false false false false false false false false false 1 false true false false 65600000 65.6 false false false 2 false true false false 52200000 52.2 false false false 3 false true false false 54600000 54.6 false false false xbrli:monetaryItemType monetary Excess 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 false 42 Retained earnings 2 us-gaap_RetainedEarningsAccumulatedDeficit us-gaap true credit instant No definition available. false false false false false false false false false false false false 1 false true false false 923100000 923.1 false false false 2 false true false false 854600000 854.6 false false false 3 false true false false 881000000 881.0 false false false xbrli:monetaryItemType monetary The 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 false 43 Accumulated other comprehensive loss, net 2 us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax us-gaap true credit instant No definition available. false false false false false false false false false false false label false 1 false true false false -14400000 -14.4 false false false 2 false true false false -13900000 -13.9 false false false 3 false true false false -11000000 -11.0 false false false xbrli:monetaryItemType monetary Accumulated 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 false 44 Total common equity 2 us-gaap_StockholdersEquity us-gaap true credit instant No definition available. false false false false false false false false false false false totallabel false 1 false true false false 1088100000 1088.1 false false false 2 false true false false 1005900000 1005.9 false false false 3 false true false false 1037700000 1037.7 false false false xbrli:monetaryItemType monetary Total 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 true 45 Total capitalization 2 us-gaap_CapitalizationLongtermDebtAndEquity us-gaap true credit instant No definition available. false false false false false false false false false false false totallabel false 1 false true false false 1511500000 1511.5 false false false 2 false true false false 1504600000 1504.6 false false false 3 false true false false 1536000000 1536.0 false false false xbrli:monetaryItemType monetary This 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. true 46 Total liabilities and capitalization 2 us-gaap_LiabilitiesAndStockholdersEquity us-gaap true credit instant No definition available. false false false false false false false false false false false totallabel false 1 false true false false 4127500000 4127.5 false false false 2 false true false false 4187600000 4187.6 false false false 3 false true false false 4435700000 4435.7 false false false xbrli:monetaryItemType monetary Total 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 true 47 Gas Distributions Equipment [Member] 0 na true na na No definition available. false true false false false false false false false false false http://nicor.com/role/condensedconsolidatedbalancesheetsunaudited false 1 false false false false 0 0 false false false 2 false false false false 0 0 false false false 3 false false false false 0 0 false false false false 5 USD true false false false us-gaap_GasDistributionEquipmentMember us-gaap_PropertyPlantAndEquipmentByTypeAxis xbrldi http://xbrl.org/2006/xbrldi us-gaap_GasDistributionEquipmentMember us-gaap_PropertyPlantAndEquipmentByTypeAxis explicitMember u000 Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 $ false 6 USD true false false false us-gaap_GasDistributionEquipmentMember us-gaap_PropertyPlantAndEquipmentByTypeAxis xbrldi http://xbrl.org/2006/xbrldi us-gaap_GasDistributionEquipmentMember us-gaap_PropertyPlantAndEquipmentByTypeAxis explicitMember u000 Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 $ false 7 USD true false false false us-gaap_GasDistributionEquipmentMember us-gaap_PropertyPlantAndEquipmentByTypeAxis xbrldi http://xbrl.org/2006/xbrldi us-gaap_GasDistributionEquipmentMember us-gaap_PropertyPlantAndEquipmentByTypeAxis explicitMember u000 Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 $ na No definition available. No authoritative reference available. false 57 Property, plant and equipment, at cost 2 us-gaap_PropertyPlantAndEquipmentGrossAbstract us-gaap true na duration No definition available. false false false false false true false false false false false terselabel false 1 false false false false 0 0 false false false 2 false false false false 0 0 false false false 3 false false false false 0 0 false false false xbrli:stringItemType string No definition available. false 58 Property, plant and equipment, at cost 2 us-gaap_PropertyPlantAndEquipmentGross us-gaap true debit instant No definition available. false false false false false false false false false false false false 1 false true false false 4646100000 4646.1 false false false 2 false true false false 4536800000 4536.8 false false false 3 false true false false 4598200000 4598.2 false false false xbrli:monetaryItemType monetary Carrying 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 false 93 Shipping [Member] 0 na true na na No definition available. false true false false false false false false false false false http://nicor.com/role/condensedconsolidatedbalancesheetsunaudited false 1 false false false false 0 0 false false false 2 false false false false 0 0 false false false 3 false false false false 0 0 false false false false 8 USD true false false false gas_ShippingMember us-gaap_PropertyPlantAndEquipmentByTypeAxis xbrldi http://xbrl.org/2006/xbrldi gas_ShippingMember us-gaap_PropertyPlantAndEquipmentByTypeAxis explicitMember u000 Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 $ false 9 USD true false false false gas_ShippingMember us-gaap_PropertyPlantAndEquipmentByTypeAxis xbrldi http://xbrl.org/2006/xbrldi gas_ShippingMember us-gaap_PropertyPlantAndEquipmentByTypeAxis explicitMember u000 Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 $ false 10 USD true false false false gas_ShippingMember us-gaap_PropertyPlantAndEquipmentByTypeAxis xbrldi http://xbrl.org/2006/xbrldi gas_ShippingMember us-gaap_PropertyPlantAndEquipmentByTypeAxis explicitMember u000 Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 $ na No definition available. No authoritative reference available. false 103 Property, plant and equipment, at cost 2 us-gaap_PropertyPlantAndEquipmentGrossAbstract us-gaap true na duration No definition available. false false false false false true false false false false false terselabel false 1 false false false false 0 0 false false false 2 false false false false 0 0 false false false 3 false false false false 0 0 false false false xbrli:stringItemType string No definition available. false 104 Property, plant and equipment, at cost 2 us-gaap_PropertyPlantAndEquipmentGross us-gaap true debit instant No definition available. false false false false false false false false false false false false 1 false true false false 338400000 338.4 false false false 2 false true false false 323200000 323.2 false false false 3 false true false false 330000000 330.0 false false false xbrli:monetaryItemType monetary Carrying 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 false 139 Other Property Plant and Equipment [Member] 0 na true na na No definition available. false true false false false false false false false false false http://nicor.com/role/condensedconsolidatedbalancesheetsunaudited false 1 false false false false 0 0 false false false 2 false false false false 0 0 false false false 3 false false false false 0 0 false false false false 11 USD true false false false gas_OtherPropertyPlantAndEquipmentMember us-gaap_PropertyPlantAndEquipmentByTypeAxis xbrldi http://xbrl.org/2006/xbrldi gas_OtherPropertyPlantAndEquipmentMember us-gaap_PropertyPlantAndEquipmentByTypeAxis explicitMember u000 Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 $ false 12 USD true false false false gas_OtherPropertyPlantAndEquipmentMember us-gaap_PropertyPlantAndEquipmentByTypeAxis xbrldi http://xbrl.org/2006/xbrldi gas_OtherPropertyPlantAndEquipmentMember us-gaap_PropertyPlantAndEquipmentByTypeAxis explicitMember u000 Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 $ false 13 USD true false false false gas_OtherPropertyPlantAndEquipmentMember us-gaap_PropertyPlantAndEquipmentByTypeAxis xbrldi http://xbrl.org/2006/xbrldi gas_OtherPropertyPlantAndEquipmentMember us-gaap_PropertyPlantAndEquipmentByTypeAxis explicitMember u000 Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 $ na No definition available. No authoritative reference available. false 149 Property, plant and equipment, at cost 2 us-gaap_PropertyPlantAndEquipmentGrossAbstract us-gaap true na duration No definition available. false false false false false true false false false false false terselabel false 1 false false false false 0 0 false false false 2 false false false false 0 0 false false false 3 false false false false 0 0 false false false xbrli:stringItemType string No definition available. false 150 Property, plant and equipment, at cost 2 us-gaap_PropertyPlantAndEquipmentGross us-gaap true debit instant No definition available. false false false false false false false false false false false false 1 true true false false 44900000 44.9 false false false 2 true true false false 29300000 29.3 false false false 3 true true false false 32800000 32.8 false false false xbrli:monetaryItemType monetary Carrying 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 false 3 54 false HundredThousands UnKnown UnKnown false true