Label	Element	Value
Southern Company Gas [Member]
Proceeds from (Payments for) Other Financing Activities	us-gaap_ProceedsFromPaymentsForOtherFinancingActivities	$ (8,000,000)
Payment for Pension and Other Postretirement Benefits	us-gaap_PensionAndOtherPostretirementBenefitContributions	125,000,000
Payments for (Proceeds from) Removal Costs	us-gaap_PaymentsForProceedsFromRemovalCosts	40,000,000
Increase (Decrease) in Other Current Liabilities	us-gaap_IncreaseDecreaseInOtherCurrentLiabilities	24,000,000
Increase (Decrease) in Other Operating Assets	us-gaap_IncreaseDecreaseInOtherOperatingAssets	31,000,000
Change in Construction Payables	so_ChangeInConstructionPayables	22,000,000
Increase (Decrease) in Accrued Salaries	us-gaap_IncreaseDecreaseInAccruedSalaries	(13,000,000)
Pension and Other Postretirement Benefits Cost (Reversal of Cost)	us-gaap_PensionAndOtherPostretirementBenefitExpense	6,000,000
Other Noncash Income (Expense)	us-gaap_OtherNoncashIncomeExpense	78,000,000
Net Cash Provided by (Used in) Investing Activities	us-gaap_NetCashProvidedByUsedInInvestingActivities	(2,067,000,000)
Deferred Income Tax Expense (Benefit)	us-gaap_DeferredIncomeTaxExpenseBenefit	92,000,000
Net Cash Provided by (Used in) Financing Activities	us-gaap_NetCashProvidedByUsedInFinancingActivities	2,399,000,000
Cash and Cash Equivalents, Period Increase (Decrease)	us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease	4,000,000
Proceeds from Equity Method Investment, Distribution, Return of Capital	us-gaap_ProceedsFromEquityMethodInvestmentDividendsOrDistributionsReturnOfCapital	5,000,000
Goodwill, Impairment Loss	us-gaap_GoodwillImpairmentLoss	0
Stock or Unit Option Plan Expense	us-gaap_StockOptionPlanExpense	20,000,000
Proceeds from Issuance of Senior Long-term Debt	us-gaap_ProceedsFromIssuanceOfSeniorLongTermDebt	900,000,000
Proceeds from (Payments to) Noncontrolling Interests	us-gaap_ProceedsFromPaymentsToMinorityShareholders	(160,000,000)
Payments for (Proceeds from) Other Investing Activities	us-gaap_PaymentsForProceedsFromOtherInvestingActivities	(4,000,000)
Proceeds from (Repayments of) Short-term Debt	us-gaap_ProceedsFromRepaymentsOfShortTermDebt	1,143,000,000
Increase Decrease in Accrued Taxes	so_IncreaseDecreaseInAccruedTaxes	8,000,000
Repayments of Medium-term Notes	us-gaap_RepaymentsOfMediumTermNotes	0
Increase (Decrease) in Prepaid Taxes	us-gaap_IncreaseDecreaseInPrepaidTaxes	23,000,000
Payments to Acquire Property, Plant, and Equipment	us-gaap_PaymentsToAcquirePropertyPlantAndEquipment	614,000,000
Increase (Decrease) in Accounts Payable	us-gaap_IncreaseDecreaseInAccountsPayable	194,000,000
Increase (Decrease) in Receivables	us-gaap_IncreaseDecreaseInReceivables	490,000,000
Hedge Settlements	so_HedgeSettlements	35,000,000
Net Cash Provided by (Used in) Operating Activities	us-gaap_NetCashProvidedByUsedInOperatingActivities	(328,000,000)
Payments of Ordinary Dividends, Common Stock	us-gaap_PaymentsOfDividendsCommonStock	126,000,000
Increase (Decrease) in Energy Related Inventory, Natural Gas in Storage	so_IncreaseDecreaseinEnergyRelatedInventoryNaturalGasinStorage	226,000,000
Proceeds from Issuance of First Mortgage Bond	us-gaap_ProceedsFromIssuanceOfFirstMortgageBond	0
Southern Company Gas [Member] | Predecessor [Member]
Proceeds from (Payments for) Other Financing Activities	us-gaap_ProceedsFromPaymentsForOtherFinancingActivities	10,000,000
Payment for Pension and Other Postretirement Benefits	us-gaap_PensionAndOtherPostretirementBenefitContributions	0
Payments for (Proceeds from) Removal Costs	us-gaap_PaymentsForProceedsFromRemovalCosts	32,000,000
Increase (Decrease) in Other Current Liabilities	us-gaap_IncreaseDecreaseInOtherCurrentLiabilities	(30,000,000)
Increase (Decrease) in Other Operating Assets	us-gaap_IncreaseDecreaseInOtherOperatingAssets	(37,000,000)
Change in Construction Payables	so_ChangeInConstructionPayables	(7,000,000)
Increase (Decrease) in Accrued Salaries	us-gaap_IncreaseDecreaseInAccruedSalaries	(21,000,000)
Pension and Other Postretirement Benefits Cost (Reversal of Cost)	us-gaap_PensionAndOtherPostretirementBenefitExpense	5,000,000
Other Noncash Income (Expense)	us-gaap_OtherNoncashIncomeExpense	82,000,000
Net Cash Provided by (Used in) Investing Activities	us-gaap_NetCashProvidedByUsedInInvestingActivities	(559,000,000)
Proceeds from Contributions from Parent	us-gaap_ProceedsFromContributionsFromParent	0
Deferred Income Tax Expense (Benefit)	us-gaap_DeferredIncomeTaxExpenseBenefit	8,000,000
Net Cash Provided by (Used in) Financing Activities	us-gaap_NetCashProvidedByUsedInFinancingActivities	(558,000,000)
Repayments of Senior Debt	us-gaap_RepaymentsOfSeniorDebt	0
Cash and Cash Equivalents, Period Increase (Decrease)	us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease	(4,000,000)
Proceeds from Equity Method Investment, Distribution, Return of Capital	us-gaap_ProceedsFromEquityMethodInvestmentDividendsOrDistributionsReturnOfCapital	3,000,000
Repayments of First Mortgage Bond	us-gaap_RepaymentsOfFirstMortgageBond	125,000,000
Goodwill, Impairment Loss	us-gaap_GoodwillImpairmentLoss	0
Stock or Unit Option Plan Expense	us-gaap_StockOptionPlanExpense	20,000,000
Proceeds from Issuance of Senior Long-term Debt	us-gaap_ProceedsFromIssuanceOfSeniorLongTermDebt	350,000,000
Proceeds from (Payments to) Noncontrolling Interests	us-gaap_ProceedsFromPaymentsToMinorityShareholders	0
Payments for (Proceeds from) Other Investing Activities	us-gaap_PaymentsForProceedsFromOtherInvestingActivities	0
Proceeds from (Repayments of) Short-term Debt	us-gaap_ProceedsFromRepaymentsOfShortTermDebt	(896,000,000)
Increase Decrease in Accrued Taxes	so_IncreaseDecreaseInAccruedTaxes	41,000,000
Repayments of Medium-term Notes	us-gaap_RepaymentsOfMediumTermNotes	0
Increase (Decrease) in Prepaid Taxes	us-gaap_IncreaseDecreaseInPrepaidTaxes	(151,000,000)
Payments to Acquire Property, Plant, and Equipment	us-gaap_PaymentsToAcquirePropertyPlantAndEquipment	509,000,000
Increase (Decrease) in Accounts Payable	us-gaap_IncreaseDecreaseInAccountsPayable	43,000,000
Increase (Decrease) in Receivables	us-gaap_IncreaseDecreaseInReceivables	(181,000,000)
Hedge Settlements	so_HedgeSettlements	26,000,000
Net Cash Provided by (Used in) Operating Activities	us-gaap_NetCashProvidedByUsedInOperatingActivities	1,113,000,000
Payments of Ordinary Dividends, Common Stock	us-gaap_PaymentsOfDividendsCommonStock	128,000,000
Increase (Decrease) in Energy Related Inventory, Natural Gas in Storage	so_IncreaseDecreaseinEnergyRelatedInventoryNaturalGasinStorage	(273,000,000)
Proceeds from Issuance of First Mortgage Bond	us-gaap_ProceedsFromIssuanceOfFirstMortgageBond	$ 250,000,000