| Label | Element | Value |
|---|---|---|
| Southern Company Gas [Member] | ||
| Interest Paid, Excluding Capitalized Interest, Operating Activities | us-gaap_InterestPaidNet | $ 135,000,000 |
| Proceeds from Divestiture of Businesses, Net of Cash Divested | us-gaap_ProceedsFromDivestitureOfBusinessesNetOfCashDivested | 0 |
| Payments of Ordinary Dividends, Common Stock | us-gaap_PaymentsOfDividendsCommonStock | 126,000,000 |
| Payment for Pension and Other Postretirement Benefits | us-gaap_PensionAndOtherPostretirementBenefitContributions | 125,000,000 |
| Income Taxes Paid, Net | us-gaap_IncomeTaxesPaidNet | 23,000,000 |
| Proceeds from (Payments for) Other Financing Activities | us-gaap_ProceedsFromPaymentsForOtherFinancingActivities | (8,000,000) |
| Payments for Removal Costs | us-gaap_PaymentsForRemovalCosts | 40,000,000 |
| Net Cash Provided by (Used in) Financing Activities | us-gaap_NetCashProvidedByUsedInFinancingActivities | 2,399,000,000 |
| Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect | us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffect | 5,000,000 |
| Deferred Income Tax Expense (Benefit) | us-gaap_DeferredIncomeTaxExpenseBenefit | 92,000,000 |
| Proceeds from Issuance of First Mortgage Bond | us-gaap_ProceedsFromIssuanceOfFirstMortgageBond | 0 |
| Repayments of Senior Debt | us-gaap_RepaymentsOfSeniorDebt | 420,000,000 |
| Net Cash Provided by (Used in) Investing Activities | us-gaap_NetCashProvidedByUsedInInvestingActivities | (2,067,000,000) |
| Repayments Of Gas Facility Revenue Bonds | so_RepaymentsOfGasFacilityRevenueBonds | 0 |
| Increase (Decrease) in Accounts Payable | us-gaap_IncreaseDecreaseInAccountsPayable | 194,000,000 |
| Proceeds from Contributions from Parent | us-gaap_ProceedsFromContributionsFromParent | 1,085,000,000 |
| Payments to Acquire Property, Plant, and Equipment | us-gaap_PaymentsToAcquirePropertyPlantAndEquipment | 614,000,000 |
| Payments for (Proceeds from) Other Investing Activities | us-gaap_PaymentsForProceedsFromOtherInvestingActivities | (4,000,000) |
| Payments to Acquire Additional Interest in Subsidiaries | us-gaap_PaymentsToAcquireAdditionalInterestInSubsidiaries | 160,000,000 |
| Change in Construction Payables | so_ChangeInConstructionPayables | 22,000,000 |
| Hedge Settlements | so_HedgeSettlements | 35,000,000 |
| Proceeds from Issuance of Senior Long-term Debt | us-gaap_ProceedsFromIssuanceOfSeniorLongTermDebt | 900,000,000 |
| Repayments of Medium-term Notes | us-gaap_RepaymentsOfMediumTermNotes | 0 |
| Proceeds from Equity Method Investment, Distribution, Return of Capital | us-gaap_ProceedsFromEquityMethodInvestmentDividendsOrDistributionsReturnOfCapital | 5,000,000 |
| Proceeds from (Repayments of) Short-term Debt | us-gaap_ProceedsFromRepaymentsOfShortTermDebt | 1,143,000,000 |
| Increase (Decrease) in Prepaid Taxes | us-gaap_IncreaseDecreaseInPrepaidTaxes | 23,000,000 |
| Net Cash Provided by (Used in) Operating Activities | us-gaap_NetCashProvidedByUsedInOperatingActivities | (327,000,000) |
| Increase (Decrease) in Accrued Salaries | us-gaap_IncreaseDecreaseInAccruedSalaries | (13,000,000) |
| Capital Expenditures Incurred but Not yet Paid | us-gaap_CapitalExpendituresIncurredButNotYetPaid | 63,000,000 |
| Increase (Decrease) in Energy Related Inventory, Natural Gas in Storage | so_IncreaseDecreaseinEnergyRelatedInventoryNaturalGasinStorage | 226,000,000 |
| Other Noncash Income (Expense) | us-gaap_OtherNoncashIncomeExpense | 51,000,000 |
| Repayments of First Mortgage Bond | us-gaap_RepaymentsOfFirstMortgageBond | 0 |
| Southern Company Gas [Member] | Predecessor [Member] | ||
| Interest Paid, Excluding Capitalized Interest, Operating Activities | us-gaap_InterestPaidNet | 119,000,000 |
| Proceeds from Divestiture of Businesses, Net of Cash Divested | us-gaap_ProceedsFromDivestitureOfBusinessesNetOfCashDivested | 0 |
| Payments of Ordinary Dividends, Common Stock | us-gaap_PaymentsOfDividendsCommonStock | 128,000,000 |
| Payment for Pension and Other Postretirement Benefits | us-gaap_PensionAndOtherPostretirementBenefitContributions | 0 |
| Income Taxes Paid, Net | us-gaap_IncomeTaxesPaidNet | (100,000,000) |
| Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents | us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalents | 19,000,000 |
| Proceeds from (Payments for) Other Financing Activities | us-gaap_ProceedsFromPaymentsForOtherFinancingActivities | 10,000,000 |
| Payments for Removal Costs | us-gaap_PaymentsForRemovalCosts | 32,000,000 |
| Net Cash Provided by (Used in) Financing Activities | us-gaap_NetCashProvidedByUsedInFinancingActivities | (558,000,000) |
| Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect | us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseExcludingExchangeRateEffect | (3,000,000) |
| Deferred Income Tax Expense (Benefit) | us-gaap_DeferredIncomeTaxExpenseBenefit | 8,000,000 |
| Proceeds from Issuance of First Mortgage Bond | us-gaap_ProceedsFromIssuanceOfFirstMortgageBond | 250,000,000 |
| Repayments of Senior Debt | us-gaap_RepaymentsOfSeniorDebt | 0 |
| Net Cash Provided by (Used in) Investing Activities | us-gaap_NetCashProvidedByUsedInInvestingActivities | (556,000,000) |
| Repayments Of Gas Facility Revenue Bonds | so_RepaymentsOfGasFacilityRevenueBonds | 0 |
| Increase (Decrease) in Accounts Payable | us-gaap_IncreaseDecreaseInAccountsPayable | 43,000,000 |
| Proceeds from Contributions from Parent | us-gaap_ProceedsFromContributionsFromParent | 0 |
| Payments to Acquire Property, Plant, and Equipment | us-gaap_PaymentsToAcquirePropertyPlantAndEquipment | 509,000,000 |
| Increase (Decrease) in Other Operating Liabilities | us-gaap_IncreaseDecreaseInOtherOperatingLiabilities | (30,000,000) |
| Payments for (Proceeds from) Other Investing Activities | us-gaap_PaymentsForProceedsFromOtherInvestingActivities | (3,000,000) |
| Payments to Acquire Additional Interest in Subsidiaries | us-gaap_PaymentsToAcquireAdditionalInterestInSubsidiaries | 0 |
| Change in Construction Payables | so_ChangeInConstructionPayables | (7,000,000) |
| Increase (Decrease) in Receivables | us-gaap_IncreaseDecreaseInReceivables | (179,000,000) |
| Hedge Settlements | so_HedgeSettlements | 26,000,000 |
| Payments to Acquire Equity Method Investments | us-gaap_PaymentsToAcquireEquityMethodInvestments | 14,000,000 |
| Proceeds from Issuance of Senior Long-term Debt | us-gaap_ProceedsFromIssuanceOfSeniorLongTermDebt | 350,000,000 |
| Repayments of Medium-term Notes | us-gaap_RepaymentsOfMediumTermNotes | 0 |
| Increase (Decrease) in Other Operating Assets | us-gaap_IncreaseDecreaseInOtherOperatingAssets | (37,000,000) |
| Proceeds from Equity Method Investment, Distribution, Return of Capital | us-gaap_ProceedsFromEquityMethodInvestmentDividendsOrDistributionsReturnOfCapital | 3,000,000 |
| Increase Decrease in Accrued Taxes | so_IncreaseDecreaseInAccruedTaxes | 41,000,000 |
| Proceeds from (Repayments of) Short-term Debt | us-gaap_ProceedsFromRepaymentsOfShortTermDebt | (896,000,000) |
| Increase (Decrease) in Prepaid Taxes | us-gaap_IncreaseDecreaseInPrepaidTaxes | (151,000,000) |
| Net Cash Provided by (Used in) Operating Activities | us-gaap_NetCashProvidedByUsedInOperatingActivities | 1,111,000,000 |
| Increase (Decrease) in Accrued Salaries | us-gaap_IncreaseDecreaseInAccruedSalaries | (21,000,000) |
| Capital Expenditures Incurred but Not yet Paid | us-gaap_CapitalExpendituresIncurredButNotYetPaid | 41,000,000 |
| Increase (Decrease) in Energy Related Inventory, Natural Gas in Storage | so_IncreaseDecreaseinEnergyRelatedInventoryNaturalGasinStorage | (273,000,000) |
| Other Noncash Income (Expense) | us-gaap_OtherNoncashIncomeExpense | 57,000,000 |
| Repayments of First Mortgage Bond | us-gaap_RepaymentsOfFirstMortgageBond | $ 125,000,000 |