1.0.0.3 false CONDENSED CONSOLIDATED STATEMENTS OF INCOME (UNAUDITED) (USD $) In Thousands false 1 $ false false Shares Standard http://www.xbrl.org/2003/instance shares xbrli 0 USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 false 2 $ false false Shares Standard http://www.xbrl.org/2003/instance shares xbrli 0 USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 false 3 $ false false Shares Standard http://www.xbrl.org/2003/instance shares xbrli 0 USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 false 4 $ false false Shares Standard http://www.xbrl.org/2003/instance shares xbrli 0 USD Standard http://www.xbrl.org/2003/iso4217 USD iso4217 0 6 Operating Revenues: 4 so_OperatingRevenues so false na duration string The abstract for the Operating Revenues subheading on the Income Statement false false false false false true false false false 1 false false 0 0 false false 2 false false 0 0 false false 3 false false 0 0 false false 4 false false 0 0 false false The abstract for the Operating Revenues subheading on the Income Statement false 7 Retail revenues 5 so_RetailRevenues so false credit duration monetary Revenues from sales of electric energy to retail regulated customers. false false false false false false false false false 1 true true 3997659000 3997659 false false 2 true true 4478292000 4478292 false false 3 true true 10355330000 10355330 false false 4 true true 10933784000 10933784 false false Revenues from sales of electric energy to retail regulated customers. No authoritative reference available. false 8 Wholesale revenues 5 so_WholesaleRevenues so false credit duration monetary Revenues from sales of energy supplied by one producer or marketer to another for eventual resale to consumers. false false false false false false false false false 1 false true 519122000 519122 false false 2 false true 774847000 774847 false false 3 false true 1408286000 1408286 false false 4 false true 1880311000 1880311 false false Revenues from sales of energy supplied by one producer or marketer to another for eventual resale to consumers. No authoritative reference available. false 9 Other electric revenues 5 so_OtherElectricRevenues so false credit duration monetary Other operating revenues of electric companies not elsewhere specified in the taxonomy. false false false false false false false false false 1 false true 139869000 139869 false false 2 false true 142459000 142459 false false 3 false true 391070000 391070 false false 4 false true 413811000 413811 false false Other operating revenues of electric companies not elsewhere specified in the taxonomy. No authoritative reference available. false 10 Other revenues 5 us-gaap_OtherSalesRevenueNet us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 24832000 24832 false false 2 false true 30901000 30901 false false 3 false true 78267000 78267 false false 4 false true 96690000 96690 false false No definition available. No authoritative reference available. true 11 Total operating revenues 5 us-gaap_Revenues us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 4681482000 4681482 false false 2 false true 5426499000 5426499 false false 3 false true 12232953000 12232953 false false 4 false true 13324596000 13324596 false false No definition available. No authoritative reference available. true 12 Operating Expenses: 4 us-gaap_OperatingExpensesAbstract us-gaap true na duration string No definition available. false false false false false true false false false 1 false false 0 0 false false 2 false false 0 0 false false 3 false false 0 0 false false 4 false false 0 0 false false No definition available. false 13 Fuel 5 us-gaap_UtilitiesOperatingExpenseFuelUsed us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 1733527000 1733527 false false 2 false true 2152828000 2152828 false false 3 false true 4588932000 4588932 false false 4 false true 5226845000 5226845 false false No definition available. No authoritative reference available. false 14 Purchased power 5 us-gaap_CostOfPurchasedPower us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 166791000 166791 false false 2 false true 378259000 378259 false false 3 false true 407623000 407623 false false 4 false true 668423000 668423 false false No definition available. No authoritative reference available. false 15 Other operations and maintenance 5 us-gaap_UtilitiesOperatingExpenseMaintenanceAndOperations us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 820889000 820889 false false 2 false true 908404000 908404 false false 3 false true 2523184000 2523184 false false 4 false true 2720219000 2720219 false false No definition available. No authoritative reference available. false 16 MC Asset Recovery litigation settlement 5 us-gaap_GainLossRelatedToLitigationSettlement us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 0 0 false false 2 false true 0 0 false false 3 false true 202000000 202000 false false 4 false true 0 0 false false No definition available. No authoritative reference available. false 17 Depreciation and amortization 5 so_DepreciationAndAmortization_ExcludingNuclearFuel so false debit duration monetary The current period expense charged against earnings on long-lived, physical assets used in the normal conduct of business and... false false false false false false false false false 1 false true 332117000 332117 false false 2 false true 367014000 367014 false false 3 false true 1099216000 1099216 false false 4 false true 1069644000 1069644 false false The current period expense charged against earnings on long-lived, physical assets used in the normal conduct of business and not intended for resale to allocate or recognize the cost of assets over their useful lives; or to record the reduction in book value of an intangible asset over the benefit period of such asset. Excludes amortization of nuclear fuel. No authoritative reference available. false 18 Taxes other than income taxes 5 us-gaap_TaxesExcludingIncomeAndExciseTaxes us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 212882000 212882 false false 2 false true 215298000 215298 false false 3 false true 620851000 620851 false false 4 false true 602612000 602612 false false No definition available. No authoritative reference available. true 19 Total operating expenses 5 us-gaap_CostsAndExpenses us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 3266206000 3266206 false false 2 false true 4021803000 4021803 false false 3 false true 9441806000 9441806 false false 4 false true 10287743000 10287743 false false No definition available. No authoritative reference available. true 20 Operating Income 4 us-gaap_OperatingIncomeLoss us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 1415276000 1415276 false false 2 false true 1404696000 1404696 false false 3 false true 2791147000 2791147 false false 4 false true 3036853000 3036853 false false No definition available. No authoritative reference available. false 21 Other Income and (Expense): 4 so_OtherIncomeAndExpense so false na duration string The abstract for the Operating Revenues subheading on the Income Statement false false false false false true false false false 1 false false 0 0 false false 2 false false 0 0 false false 3 false false 0 0 false false 4 false false 0 0 false false The abstract for the Operating Revenues subheading on the Income Statement false 22 Allowance for equity funds used during construction 5 so_AllowanceForEquityFundsUsedDuringConstruction so false credit duration monetary Total increase in earnings in the period representing the cost of equity (rate of return) used to finance construction of... false false false false false false false false false 1 false true 51061000 51061 false false 2 false true 35541000 35541 false false 3 false true 141173000 141173 false false 4 false true 111612000 111612 false false Total increase in earnings in the period representing the cost of equity (rate of return) used to finance construction of regulated assets, which is expected to be recovered through rate adjustments. No authoritative reference available. false 23 Interest income 5 us-gaap_InvestmentIncomeInterestAndDividend us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 6013000 6013 false false 2 false true 9744000 9744 false false 3 false true 17791000 17791 false false 4 false true 20737000 20737 false false No definition available. No authoritative reference available. false 24 Equity in income (losses) of unconsolidated subsidiaries 5 us-gaap_IncomeLossFromEquityMethodInvestments us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true -34000 -34 false false 2 false true 4704000 4704 false false 3 false true -330000 -330 false false 4 false true 6129000 6129 false false No definition available. No authoritative reference available. false 25 Leveraged lease income (losses) 5 us-gaap_LeveragedLeasesIncomeStatementNetIncomeFromLeveragedLeases us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 6578000 6578 false false 2 false true 6343000 6343 false false 3 false true 24695000 24695 false false 4 false true -53611000 -53611 false false No definition available. No authoritative reference available. false 26 Gain on disposition of lease termination 5 us-gaap_GainLossOnContractTermination us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 0 0 false false 2 false true 0 0 false false 3 false true 26300000 26300 false false 4 false true 0 0 false false No definition available. No authoritative reference available. false 27 Loss on extinguishment of debt 5 us-gaap_GainsLossesOnExtinguishmentOfDebtBeforeWriteOffOfDeferredDebtIssuanceCost us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 0 0 false false 2 false true 0 0 false false 3 false true -17184000 -17184 false false 4 false true 0 0 false false No definition available. No authoritative reference available. false 28 Interest expense, net of amounts capitalized 5 us-gaap_InterestAndDebtExpense us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true -226345000 -226345 false false 2 false true -219066000 -219066 false false 3 false true -684902000 -684902 false false 4 false true -665123000 -665123 false false No definition available. No authoritative reference available. false 29 Other income (expense), net 5 us-gaap_OtherNonoperatingIncomeExpense us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true -10432000 -10432 false false 2 false true -10816000 -10816 false false 3 false true -26963000 -26963 false false 4 false true -14385000 -14385 false false No definition available. No authoritative reference available. true 30 Total other income and (expense) 5 us-gaap_NonoperatingIncomeExpense us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true -173159000 -173159 false false 2 false true -173550000 -173550 false false 3 false true -519420000 -519420 false false 4 false true -594641000 -594641 false false No definition available. No authoritative reference available. true 31 Earnings Before Income Taxes 4 us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 1242117000 1242117 false false 2 false true 1231146000 1231146 false false 3 false true 2271727000 2271727 false false 4 false true 2442212000 2442212 false false No definition available. No authoritative reference available. false 32 Income taxes 4 us-gaap_IncomeTaxExpenseBenefit us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 435947000 435947 false false 2 false true 434515000 434515 false false 3 false true 828833000 828833 false false 4 false true 837605000 837605 false false No definition available. No authoritative reference available. true 33 Consolidated net income 4 us-gaap_ProfitLoss us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 false true 806170000 806170 false false 2 false true 796631000 796631 false false 3 false true 1442894000 1442894 false false 4 false true 1604607000 1604607 false false No definition available. No authoritative reference available. false 34 Dividends on Preferred and Preference Stock of Subsidiaries 4 us-gaap_NetIncomeLossAttributableToNoncontrollingInterest us-gaap true debit duration monetary No definition available. false false false false false false false false false 1 false true 16195000 16195 false false 2 false true 16195000 16195 false false 3 false true 48585000 48585 false false 4 false true 48585000 48585 false false No definition available. No authoritative reference available. true 35 Consolidated Net Income After Dividends on Preferred and Preference Stock of Subsidiaries 4 us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic us-gaap true credit duration monetary No definition available. false false false false false false false false false 1 true true 789975000 789975 false false 2 true true 780436000 780436 false false 3 true true 1394309000 1394309 false false 4 true true 1556022000 1556022 false false No definition available. No authoritative reference available. true 37 Earnings per share (EPS) - 5 us-gaap_EarningsPerShareAbstract us-gaap true na duration string No definition available. false false false false false true false false false 1 false false 0 0 false false 2 false false 0 0 false false 3 false false 0 0 false false 4 false false 0 0 false false No definition available. false 38 Basic EPS 6 us-gaap_EarningsPerShareBasic us-gaap true na duration decimal No definition available. false false false false false false false false false 1 false true 0.99 0.99 false false 2 false true 1.01 1.01 false false 3 false true 1.77 1.77 false false 4 false true 2.02 2.02 false false No definition available. No authoritative reference available. false 39 Diluted EPS 6 us-gaap_EarningsPerShareDiluted us-gaap true na duration decimal No definition available. false false false false false false false false false 1 false true 0.99 0.99 false false 2 false true 1 1 false false 3 false true 1.76 1.76 false false 4 false true 2.01 2.01 false false No definition available. No authoritative reference available. false 40 Average number of shares of common stock outstanding (in thousands) 5 so_AverageNumberOfSharesOfCommonStockOutstandingInThousands so false na duration string The abstract for the average number of shares of common stock outstanding subheading on the Income Statement. false false false false false true false false false 1 false false 0 0 false false 2 false false 0 0 false false 3 false false 0 0 false false 4 false false 0 0 false false The abstract for the average number of shares of common stock outstanding subheading on the Income Statement. false 41 Basic 6 us-gaap_WeightedAverageNumberOfSharesOutstandingBasic us-gaap true na duration shares No definition available. false false false false false false false false false 1 false true 798418000 798418 false false 2 false true 772622000 772622 false false 3 false true 789675000 789675 false false 4 false true 769298000 769298 false false No definition available. No authoritative reference available. false 42 Diluted 6 us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding us-gaap true na duration shares No definition available. false false false false false false false false false 1 false true 800178000 800178 false false 2 false true 776903000 776903 false false 3 false true 791259000 791259 false false 4 false true 773451000 773451 false false No definition available. No authoritative reference available. false 43 Cash dividends paid per share of common stock 5 us-gaap_CommonStockDividendsPerShareCashPaid us-gaap true na duration decimal No definition available. false false false false false false false false false 1 false true 0.4375 0.4375 false false 2 false true 0.42 0.42 false false 3 false true 1.295 1.295 false false 4 false true 1.2425 1.2425 false false No definition available. No authoritative reference available. false false 4 37 false Thousands Thousands UnKnown false true