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or services (including trade receivables) that have been delivered or sold in the normal course of business.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.3) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 3 -Subparagraph a, b -Article 5 false2Customer accounts receivable82false 7so_UnbilledRevenuesCurrentso_falsedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse160000000160000000USD$falsefalsefalse2truefalsefalse138000000138000000USD$falsefalsefalsexbrli:monetaryItemTypemonetaryCarrying amount at the balance sheet date of revenues which have been earned but not yet billed.No definition available.false2Unbilled revenues83false 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http://asc.fasb.org/extlink&oid=28358313&loc=d3e6676-107765 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph k -Subparagraph 2 -Article 4 false2Affiliated companies86false 7us-gaap_AllowanceForDoubtfulAccountsReceivableCurrentus-gaap_truecreditinstantfalsefalsefalsefalsefalsefalsefalsetruenegatedLabel1truefalsefalse-9000000-9000000USD$falsefalsefalse2truefalsefalse-8000000-8000000USD$falsefalsefalsexbrli:monetaryItemTypemonetaryA valuation allowance for trade and other receivables due to an Entity within one year (or the normal operating cycle, whichever is longer) that are expected to be uncollectible.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 4 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=28368275&loc=d3e5074-111524 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.4) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 false2Accumulated provision for uncollectible accounts87false 6us-gaap_EnergyRelatedInventoryOtherFossilFuelus-gaap_truedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse399000000399000000USD$falsefalsefalse2truefalsefalse475000000475000000USD$falsefalsefalsexbrli:monetaryItemTypemonetaryCarrying amount as of the balance sheet date of other types of fuel derived from living matter of a previous geologic time, not otherwise itemized.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.6(a)) 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-URI http://asc.fasb.org/extlink&oid=6516093 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.6(a)(5)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 false2Materials and supplies, at average cost89false 6so_VacationPayso_falsedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse6100000061000000USD$falsefalsefalse2truefalsefalse6100000061000000USD$falsefalsefalsexbrli:monetaryItemTypemonetaryCarrying amount at the balance sheet date of the regulatory asset related to vacation pay to employees.No definition available.false2Vacation pay90false 6us-gaap_PrepaidExpenseCurrentus-gaap_truedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse9700000097000000USD$falsefalsefalse2truefalsefalse8100000081000000USD$falsefalsefalsexbrli:monetaryItemTypemonetaryAmount of asset related to consideration paid in advance for costs that provide economic benefits within a future period of one year or the normal operating cycle, if longer.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Current Assets -URI http://asc.fasb.org/extlink&oid=6509628 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=28358313&loc=d3e6676-107765 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=28358313&loc=d3e6787-107765 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 340 -SubTopic 10 -Section 05 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6386993&loc=d3e5879-108316 false2Prepaid expenses91false 6so_OtherRegulatoryAssetsCurrentso_falsedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse1600000016000000USD$falsefalsefalse2truefalsefalse2400000024000000USD$falsefalsefalsexbrli:monetaryItemTypemonetaryCarrying amount of other regulatory assets (not elsewhere included) as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. 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Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.13) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 false2In service96false 6us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipmentus-gaap_truecreditinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse79160000007916000000USD$falsefalsefalse2truefalsefalse77610000007761000000USD$falsefalsefalsexbrli:monetaryItemTypemonetaryAmount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.14) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 14 -Article 5 false2Less accumulated provision for depreciation97false 6so_PlantInServiceNetOfDepreciationso_falsedebitinstantfalsefalsefalsefalsefalsefalsefalsefalsetotalLabel1truefalsefalse1375800000013758000000USD$falsefalsefalse2truefalsefalse1364600000013646000000USD$falsefalsefalsexbrli:monetaryItemTypemonetaryCarrying amount at the balance sheet date for long-lived physical assets used in the normal conduct of business and not intended for resale, less depreciation.No definition available.true2Plant in service, net of depreciation98false 6so_NuclearFuelAtAmortizedCostso_falsedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse366000000366000000USD$falsefalsefalse2truefalsefalse354000000354000000USD$falsefalsefalsexbrli:monetaryItemTypemonetaryThe carrying amount as of the balance sheet of nuclear fuel held as property plant and equipment. This amount is net of amortization based on the energy produced over the life of the fuel.No definition available.false2Nuclear fuel, at amortized cost99false 6us-gaap_ConstructionInProgressGrossus-gaap_truedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse555000000555000000USD$falsefalsefalse2truefalsefalse438000000438000000USD$falsefalsefalsexbrli:monetaryItemTypemonetaryAmount of structure or a modification to a structure under construction. Includes recently completed structures or modifications to structures that have not been placed into service.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 false2Construction work in progress100false 6us-gaap_PropertyPlantAndEquipmentNetus-gaap_truedebitinstantfalsefalsefalsefalsefalsefalsefalsefalsetotalLabel1truefalsefalse1467900000014679000000USD$falsefalsefalse2truefalsefalse1443800000014438000000USD$falsefalsefalsexbrli:monetaryItemTypemonetaryAmount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.13) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: 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 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 8 -Article 7 true2Total property, plant, and equipment101true 5us-gaap_LongTermInvestmentsAbstractus-gaap_truenadurationfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1falsefalsefalse00falsefalsefalse2falsefalsefalse00falsefalsefalsexbrli:stringItemTypestringfalse0Other Property and Investments:102false 6us-gaap_InvestmentsInAffiliatesSubsidiariesAssociatesAndJointVenturesus-gaap_truedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse5500000055000000USD$falsefalsefalse2truefalsefalse5300000053000000USD$falsefalsefalsexbrli:monetaryItemTypemonetaryTotal investments in (A) an entity in which the entity has significant influence, but does not have control, (B) subsidiaries that are not required to be consolidated and are accounted for using the equity and or cost method, and (C) an entity in which the reporting entity shares control of the entity with another party or group. Includes long-term advances receivable from a party that is affiliated with the reporting entity by means of direct or indirect ownership.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.12) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 false2Equity investments in unconsolidated subsidiaries103false 6us-gaap_DecommissioningFundInvestmentsus-gaap_truedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse644000000644000000USD$falsefalsefalse2truefalsefalse605000000605000000USD$falsefalsefalsexbrli:monetaryItemTypemonetaryDecommission fund to pay for the costs of decontaminating and decommissioning of facilities through collection of revenues derived from utility assessments and government appropriations. Decommission fund investment for the process whereby a power station, at the end of its economic life, is taken permanently out of service and its site made available for other purposes. In the case of a nuclear station this comprises three different states of clearance. 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http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 08 -Paragraph k -Subparagraph 2 -Article 4 false2Affiliated companies159false 7us-gaap_AllowanceForDoubtfulAccountsReceivableCurrentus-gaap_truecreditinstantfalsefalsefalsefalsefalsefalsefalsetruenegatedLabel1truefalsefalse-6000000-6000000USD$falsefalsefalse2truefalsefalse-6000000-6000000USD$falsefalsefalsexbrli:monetaryItemTypemonetaryA valuation allowance for trade and other receivables due to an Entity within one year (or the normal operating cycle, whichever is longer) that are expected to be uncollectible.Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 4 -Article 5 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=28368275&loc=d3e5074-111524 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http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (g)(4) -URI http://asc.fasb.org/extlink&oid=28358313&loc=d3e6676-107765 false2Prepaid income taxes164false 6so_OtherRegulatoryAssetsCurrentso_falsedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse4600000046000000USD$falsefalsefalse2truefalsefalse7200000072000000USD$falsefalsefalsexbrli:monetaryItemTypemonetaryCarrying amount of other regulatory assets (not elsewhere included) as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. 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http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 340 -SubTopic 10 -Section 05 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=6386993&loc=d3e5865-108316 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 340 -SubTopic 10 -Section 05 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6386993&loc=d3e5879-108316 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 45 -Paragraph 1 -Subparagraph (g)(4) -URI http://asc.fasb.org/extlink&oid=28358313&loc=d3e6676-107765 false2Prepaid income taxes299false 6so_OtherRegulatoryAssetsCurrentso_falsedebitinstantfalsefalsefalsefalsefalsefalsefalsefalseverboseLabel1truefalsefalse5941800059418000USD$falsefalsefalse2truefalsefalse5530200055302000USD$falsefalsefalsexbrli:monetaryItemTypemonetaryCarrying amount of other regulatory 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