Label	Element	Value
Southern Company Gas [Member]
Proceeds from (Payments for) Other Financing Activities	us-gaap_ProceedsFromPaymentsForOtherFinancingActivities	$ (8,000,000)
Increase (Decrease) in Prepaid Taxes	us-gaap_IncreaseDecreaseInPrepaidTaxes	(1,000,000)
Payments to Acquire Equity Method Investments	us-gaap_PaymentsToAcquireEquityMethodInvestments	1,421,000,000
Deferred Income Tax Expense (Benefit)	us-gaap_DeferredIncomeTaxExpenseBenefit	(30,000,000)
Repayments of Senior Debt	us-gaap_RepaymentsOfSeniorDebt	300,000,000
Depreciation, Amortization and Accretion, Net	us-gaap_DepreciationAmortizationAndAccretionNet	116,000,000
Pension and Other Postretirement Benefits Cost (Reversal of Cost)	us-gaap_PensionAndOtherPostretirementBenefitExpense	(123,000,000)
Increase (Decrease) in Accounts Payable	us-gaap_IncreaseDecreaseInAccountsPayable	78,000,000
Increase Decrease in Accrued Taxes	so_IncreaseDecreaseInAccruedTaxes	(11,000,000)
Stock or Unit Option Plan Expense	us-gaap_StockOptionPlanExpense	11,000,000
Interest Paid, Net	us-gaap_InterestPaidNet	86,000,000
Repayments of First Mortgage Bond	us-gaap_RepaymentsOfFirstMortgageBond	0
Increase (Decrease) in Other Operating Assets	us-gaap_IncreaseDecreaseInOtherOperatingAssets	36,000,000
Payments for (Proceeds from) Removal Costs	us-gaap_PaymentsForProceedsFromRemovalCosts	21,000,000
Proceeds from Issuance of First Mortgage Bond	us-gaap_ProceedsFromIssuanceOfFirstMortgageBond	0
Proceeds from Equity Method Investment, Distribution, Return of Capital	us-gaap_ProceedsFromEquityMethodInvestmentDividendsOrDistributionsReturnOfCapital	2,000,000
Cash and Cash Equivalents, Period Increase (Decrease)	us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease	33,000,000
Net Cash Provided by (Used in) Financing Activities	us-gaap_NetCashProvidedByUsedInFinancingActivities	2,090,000,000
Income Taxes Paid, Net	us-gaap_IncomeTaxesPaidNet	54,000,000
Increase (Decrease) in Receivables	us-gaap_IncreaseDecreaseInReceivables	18,000,000
Proceeds from Issuance of Senior Long-term Debt	us-gaap_ProceedsFromIssuanceOfSeniorLongTermDebt	900,000,000
Net Cash Provided by (Used in) Operating Activities	us-gaap_NetCashProvidedByUsedInOperatingActivities	(342,000,000)
Hedge Settlements	so_HedgeSettlements	35,000,000
Other Noncash Income (Expense)	us-gaap_OtherNoncashIncomeExpense	47,000,000
Increase (Decrease) in Energy Related Inventory, Natural Gas in Storage	so_IncreaseDecreaseinEnergyRelatedInventoryNaturalGasinStorage	222,000,000
Payments to Acquire Property, Plant, and Equipment	us-gaap_PaymentsToAcquirePropertyPlantAndEquipment	287,000,000
Change in Construction Payables	so_ChangeInConstructionPayables	9,000,000
Repayments of Long-term Debt	us-gaap_RepaymentsOfLongTermDebt	0
Payments to Noncontrolling Interests	us-gaap_PaymentsToMinorityShareholders	0
Increase (Decrease) in Other Operating Liabilities	us-gaap_IncreaseDecreaseInOtherOperatingLiabilities	(11,000,000)
Payments of Ordinary Dividends, Common Stock	us-gaap_PaymentsOfDividendsCommonStock	63,000,000
Proceeds from (Repayments of) Short-term Debt	us-gaap_ProceedsFromRepaymentsOfShortTermDebt	472,000,000
Increase (Decrease) in Accrued Salaries	us-gaap_IncreaseDecreaseInAccruedSalaries	(36,000,000)
Payments for (Proceeds from) Other Investing Activities	us-gaap_PaymentsForProceedsFromOtherInvestingActivities	(3,000,000)
Capital Expenditures Incurred but Not yet Paid	us-gaap_CapitalExpendituresIncurredButNotYetPaid	50,000,000
Net Cash Provided by (Used in) Investing Activities	us-gaap_NetCashProvidedByUsedInInvestingActivities	(1,715,000,000)
Southern Company Gas [Member] | Predecessor [Member]
Proceeds from (Payments for) Other Financing Activities	us-gaap_ProceedsFromPaymentsForOtherFinancingActivities	10,000,000
Increase (Decrease) in Prepaid Taxes	us-gaap_IncreaseDecreaseInPrepaidTaxes	(151,000,000)
Payments to Acquire Equity Method Investments	us-gaap_PaymentsToAcquireEquityMethodInvestments	14,000,000
Deferred Income Tax Expense (Benefit)	us-gaap_DeferredIncomeTaxExpenseBenefit	8,000,000
Repayments of Senior Debt	us-gaap_RepaymentsOfSeniorDebt	0
Depreciation, Amortization and Accretion, Net	us-gaap_DepreciationAmortizationAndAccretionNet	206,000,000
Pension and Other Postretirement Benefits Cost (Reversal of Cost)	us-gaap_PensionAndOtherPostretirementBenefitExpense	5,000,000
Increase (Decrease) in Accounts Payable	us-gaap_IncreaseDecreaseInAccountsPayable	43,000,000
Increase Decrease in Accrued Taxes	so_IncreaseDecreaseInAccruedTaxes	41,000,000
Stock or Unit Option Plan Expense	us-gaap_StockOptionPlanExpense	20,000,000
Interest Paid, Net	us-gaap_InterestPaidNet	119,000,000
Repayments of First Mortgage Bond	us-gaap_RepaymentsOfFirstMortgageBond	125,000,000
Increase (Decrease) in Other Operating Assets	us-gaap_IncreaseDecreaseInOtherOperatingAssets	(37,000,000)
Payments for (Proceeds from) Removal Costs	us-gaap_PaymentsForProceedsFromRemovalCosts	32,000,000
Proceeds from Issuance of First Mortgage Bond	us-gaap_ProceedsFromIssuanceOfFirstMortgageBond	250,000,000
Proceeds from Equity Method Investment, Distribution, Return of Capital	us-gaap_ProceedsFromEquityMethodInvestmentDividendsOrDistributionsReturnOfCapital	3,000,000
Cash and Cash Equivalents, Period Increase (Decrease)	us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease	(4,000,000)
Net Cash Provided by (Used in) Financing Activities	us-gaap_NetCashProvidedByUsedInFinancingActivities	(558,000,000)
Income Taxes Paid, Net	us-gaap_IncomeTaxesPaidNet	(100,000,000)
Proceeds from Contributions from Parent	us-gaap_ProceedsFromContributionsFromParent	0
Increase (Decrease) in Receivables	us-gaap_IncreaseDecreaseInReceivables	(181,000,000)
Proceeds from Issuance of Senior Long-term Debt	us-gaap_ProceedsFromIssuanceOfSeniorLongTermDebt	350,000,000
Net Cash Provided by (Used in) Operating Activities	us-gaap_NetCashProvidedByUsedInOperatingActivities	1,113,000,000
Cash and Cash Equivalents, at Carrying Value	us-gaap_CashAndCashEquivalentsAtCarryingValue	15,000,000
Hedge Settlements	so_HedgeSettlements	26,000,000
Other Noncash Income (Expense)	us-gaap_OtherNoncashIncomeExpense	82,000,000
Increase (Decrease) in Energy Related Inventory, Natural Gas in Storage	so_IncreaseDecreaseinEnergyRelatedInventoryNaturalGasinStorage	(273,000,000)
Payments to Acquire Property, Plant, and Equipment	us-gaap_PaymentsToAcquirePropertyPlantAndEquipment	509,000,000
Change in Construction Payables	so_ChangeInConstructionPayables	(7,000,000)
Repayments of Long-term Debt	us-gaap_RepaymentsOfLongTermDebt	0
Payments to Noncontrolling Interests	us-gaap_PaymentsToMinorityShareholders	19,000,000
Increase (Decrease) in Other Operating Liabilities	us-gaap_IncreaseDecreaseInOtherOperatingLiabilities	(30,000,000)
Payments of Ordinary Dividends, Common Stock	us-gaap_PaymentsOfDividendsCommonStock	128,000,000
Proceeds from (Repayments of) Short-term Debt	us-gaap_ProceedsFromRepaymentsOfShortTermDebt	(896,000,000)
Increase (Decrease) in Accrued Salaries	us-gaap_IncreaseDecreaseInAccruedSalaries	(21,000,000)
Payments for (Proceeds from) Other Investing Activities	us-gaap_PaymentsForProceedsFromOtherInvestingActivities	0
Capital Expenditures Incurred but Not yet Paid	us-gaap_CapitalExpendituresIncurredButNotYetPaid	41,000,000
Net Cash Provided by (Used in) Investing Activities	us-gaap_NetCashProvidedByUsedInInvestingActivities	$ (559,000,000)