Label	Element	Value
Southern Company Gas [Member] | Successor [Member]
Stock or Unit Option Plan Expense	us-gaap_StockOptionPlanExpense	$ 20,000,000
Pension and Other Postretirement Benefit Expense	us-gaap_PensionAndOtherPostretirementBenefitExpense	6,000,000
Net Cash Provided by (Used in) Continuing Operations	us-gaap_NetCashProvidedByUsedInContinuingOperations	4,000,000
Proceeds from Issuance of Senior Long-term Debt	us-gaap_ProceedsFromIssuanceOfSeniorLongTermDebt	900,000,000
Goodwill, Impairment Loss	us-gaap_GoodwillImpairmentLoss	0
Payments to Acquire Equity Method Investments	us-gaap_PaymentsToAcquireEquityMethodInvestments	1,444,000,000
Net Cash Provided by (Used in) Discontinued Operations	us-gaap_NetCashProvidedByUsedInDiscontinuedOperations	0
Deferred Income Tax Expense (Benefit)	us-gaap_DeferredIncomeTaxExpenseBenefit	92,000,000
Pension and Other Postretirement Benefit Contributions	us-gaap_PensionAndOtherPostretirementBenefitContributions	125,000,000
Net Cash Provided by (Used in) Financing Activities	us-gaap_NetCashProvidedByUsedInFinancingActivities	2,399,000,000
Proceeds from Sales of Business, Affiliate and Productive Assets	us-gaap_ProceedsFromSalesOfBusinessAffiliateAndProductiveAssets	0
Payments for (Proceeds from) Other Investing Activities	us-gaap_PaymentsForProceedsFromOtherInvestingActivities	(9,000,000)
Cash Provided by (Used in) Investing Activities, Discontinued Operations	us-gaap_CashProvidedByUsedInInvestingActivitiesDiscontinuedOperations	0
Repayments of First Mortgage Bond	us-gaap_RepaymentsOfFirstMortgageBond	0
Repayments of Senior Debt	us-gaap_RepaymentsOfSeniorDebt	420,000,000
Increase (Decrease) in Accounts Payable	us-gaap_IncreaseDecreaseInAccountsPayable	194,000,000
Net Cash Provided by (Used in) Operating Activities	us-gaap_NetCashProvidedByUsedInOperatingActivities	(328,000,000)
Increase (Decrease) in Receivables	us-gaap_IncreaseDecreaseInReceivables	490,000,000
Payments to Acquire Property, Plant, and Equipment	us-gaap_PaymentsToAcquirePropertyPlantAndEquipment	614,000,000
Other Noncash Income (Expense)	us-gaap_OtherNoncashIncomeExpense	78,000,000
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax	0
Change in Construction Payables	so_ChangeInConstructionPayables	22,000,000
Cash Provided by (Used in) Operating Activities, Discontinued Operations	us-gaap_CashProvidedByUsedInOperatingActivitiesDiscontinuedOperations	0
Increase (Decrease) in Energy Related Inventory, Natural Gas in Storage	so_IncreaseDecreaseinEnergyRelatedInventoryNaturalGasinStorage	226,000,000
Proceeds from (Payments for) Other Financing Activities	us-gaap_ProceedsFromPaymentsForOtherFinancingActivities	(8,000,000)
Increase (Decrease) in Other Current Liabilities	us-gaap_IncreaseDecreaseInOtherCurrentLiabilities	24,000,000
Increase Decrease in Accrued Taxes	so_IncreaseDecreaseInAccruedTaxes	8,000,000
Increase (Decrease) in Other Operating Assets	us-gaap_IncreaseDecreaseInOtherOperatingAssets	31,000,000
Hedge Settlements	so_HedgeSettlements	35,000,000
Proceeds from (Repayments of) Short-term Debt	us-gaap_ProceedsFromRepaymentsOfShortTermDebt	1,143,000,000
Increase (Decrease) in Accrued Salaries	us-gaap_IncreaseDecreaseInAccruedSalaries	(13,000,000)
Net Cash Provided by (Used in) Investing Activities	us-gaap_NetCashProvidedByUsedInInvestingActivities	(2,067,000,000)
Increase (Decrease) in Prepaid Taxes	us-gaap_IncreaseDecreaseInPrepaidTaxes	23,000,000
Payments for (Proceeds from) Removal Costs	us-gaap_PaymentsForProceedsFromRemovalCosts	40,000,000
Proceeds from (Payments to) Noncontrolling Interests	us-gaap_ProceedsFromPaymentsToMinorityShareholders	(160,000,000)
Payments of Ordinary Dividends, Common Stock	us-gaap_PaymentsOfDividendsCommonStock	126,000,000
Proceeds from Issuance of First Mortgage Bond	us-gaap_ProceedsFromIssuanceOfFirstMortgageBond	0
Southern Company Gas [Member] | Predecessor [Member]
Stock or Unit Option Plan Expense	us-gaap_StockOptionPlanExpense	20,000,000
Pension and Other Postretirement Benefit Expense	us-gaap_PensionAndOtherPostretirementBenefitExpense	5,000,000
Net Cash Provided by (Used in) Continuing Operations	us-gaap_NetCashProvidedByUsedInContinuingOperations	(4,000,000)
Proceeds from Contributions from Parent	us-gaap_ProceedsFromContributionsFromParent	0
Proceeds from Issuance of Senior Long-term Debt	us-gaap_ProceedsFromIssuanceOfSeniorLongTermDebt	350,000,000
Goodwill, Impairment Loss	us-gaap_GoodwillImpairmentLoss	0
Payments to Acquire Equity Method Investments	us-gaap_PaymentsToAcquireEquityMethodInvestments	14,000,000
Net Cash Provided by (Used in) Discontinued Operations	us-gaap_NetCashProvidedByUsedInDiscontinuedOperations	0
Deferred Income Tax Expense (Benefit)	us-gaap_DeferredIncomeTaxExpenseBenefit	8,000,000
Pension and Other Postretirement Benefit Contributions	us-gaap_PensionAndOtherPostretirementBenefitContributions	0
Net Cash Provided by (Used in) Financing Activities	us-gaap_NetCashProvidedByUsedInFinancingActivities	(558,000,000)
Proceeds from Sales of Business, Affiliate and Productive Assets	us-gaap_ProceedsFromSalesOfBusinessAffiliateAndProductiveAssets	0
Payments for (Proceeds from) Other Investing Activities	us-gaap_PaymentsForProceedsFromOtherInvestingActivities	(3,000,000)
Cash Provided by (Used in) Investing Activities, Discontinued Operations	us-gaap_CashProvidedByUsedInInvestingActivitiesDiscontinuedOperations	0
Repayments of First Mortgage Bond	us-gaap_RepaymentsOfFirstMortgageBond	125,000,000
Repayments of Senior Debt	us-gaap_RepaymentsOfSeniorDebt	0
Increase (Decrease) in Accounts Payable	us-gaap_IncreaseDecreaseInAccountsPayable	43,000,000
Net Cash Provided by (Used in) Operating Activities	us-gaap_NetCashProvidedByUsedInOperatingActivities	1,113,000,000
Increase (Decrease) in Receivables	us-gaap_IncreaseDecreaseInReceivables	(181,000,000)
Payments to Acquire Property, Plant, and Equipment	us-gaap_PaymentsToAcquirePropertyPlantAndEquipment	509,000,000
Other Noncash Income (Expense)	us-gaap_OtherNoncashIncomeExpense	82,000,000
Change in Construction Payables	so_ChangeInConstructionPayables	(7,000,000)
Cash Provided by (Used in) Operating Activities, Discontinued Operations	us-gaap_CashProvidedByUsedInOperatingActivitiesDiscontinuedOperations	0
Increase (Decrease) in Energy Related Inventory, Natural Gas in Storage	so_IncreaseDecreaseinEnergyRelatedInventoryNaturalGasinStorage	(273,000,000)
Proceeds from (Payments for) Other Financing Activities	us-gaap_ProceedsFromPaymentsForOtherFinancingActivities	10,000,000
Increase (Decrease) in Other Current Liabilities	us-gaap_IncreaseDecreaseInOtherCurrentLiabilities	(30,000,000)
Increase Decrease in Accrued Taxes	so_IncreaseDecreaseInAccruedTaxes	41,000,000
Increase (Decrease) in Other Operating Assets	us-gaap_IncreaseDecreaseInOtherOperatingAssets	(37,000,000)
Hedge Settlements	so_HedgeSettlements	26,000,000
Proceeds from (Repayments of) Short-term Debt	us-gaap_ProceedsFromRepaymentsOfShortTermDebt	(896,000,000)
Increase (Decrease) in Accrued Salaries	us-gaap_IncreaseDecreaseInAccruedSalaries	(21,000,000)
Net Cash Provided by (Used in) Investing Activities	us-gaap_NetCashProvidedByUsedInInvestingActivities	(559,000,000)
Increase (Decrease) in Prepaid Taxes	us-gaap_IncreaseDecreaseInPrepaidTaxes	(151,000,000)
Payments for (Proceeds from) Removal Costs	us-gaap_PaymentsForProceedsFromRemovalCosts	32,000,000
Proceeds from (Payments to) Noncontrolling Interests	us-gaap_ProceedsFromPaymentsToMinorityShareholders	0
Payments of Ordinary Dividends, Common Stock	us-gaap_PaymentsOfDividendsCommonStock	128,000,000
Proceeds from Issuance of First Mortgage Bond	us-gaap_ProceedsFromIssuanceOfFirstMortgageBond	$ 250,000,000