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Derivatives - Schedule of Pre-Tax Effects of Cash Flow and Fair Value Hedging on Income (Details) - USD ($) $ in Millions | 3 Months Ended | 9 Months Ended
--- | --- | ---
Sep. 30, 2025 | Sep. 30, 2024 | Sep. 30, 2025 | Sep. 30, 2024
Other Income (Expense) |  |  |  |
Derivative [Line Items] |  |  |  |
Amount excluded from effectiveness testing recognized in earnings | $ (1) | $ 3 | $ 12 | $ 8
Cash Flow Hedging | Energy-related derivatives | Cost of Goods and Services Sold | Fuel |  |  |  |
Derivative [Line Items] |  |  |  |
Gain (loss) on hedges | 0 | (3) | 1 | (5)
Cash Flow Hedging | Energy-related derivatives | Cost of Goods and Services Sold | Natural gas |  |  |  |
Derivative [Line Items] |  |  |  |
Gain (loss) on hedges | (1) | (4) | (1) | (34)
Cash Flow Hedging | Energy-related derivatives | Operating Expense |  |  |  |
Derivative [Line Items] |  |  |  |
Gain (loss) on hedges | 0 | 0 | 0 | (1)
Cash Flow Hedging | Interest rate derivatives | Interest Expense |  |  |  |
Derivative [Line Items] |  |  |  |
Gain (loss) on hedges | (4) | (4) | (10) | (12)
Cash Flow Hedging | Foreign currency derivatives | Interest Expense |  |  |  |
Derivative [Line Items] |  |  |  |
Gain (loss) on hedges | (2) | (3) | (8) | (9)
Cash Flow Hedging | Foreign currency derivatives | Other Income (Expense) |  |  |  |
Derivative [Line Items] |  |  |  |
Gain (loss) on hedges | 1 | 24 | 68 | 7
Fair Value Hedging | Interest rate derivatives | Interest Expense |  |  |  |
Derivative [Line Items] |  |  |  |
Gain (loss) on hedges | 16 | 78 | 85 | 47
Fair Value Hedging | Foreign currency derivatives |  |  |  |
Derivative [Line Items] |  |  |  |
Amount excluded from effectiveness testing recognized in earnings | 1 | (2) | (12) | (8)
Fair Value Hedging | Foreign currency derivatives | Other Income (Expense) |  |  |  |
Derivative [Line Items] |  |  |  |
Gain (loss) on hedges | 5 | 58 | 160 | 79
SOUTHERN POWER CO | Cash Flow Hedging | Energy-related derivatives | Cost of Goods and Services Sold | Fuel |  |  |  |
Derivative [Line Items] |  |  |  |
Gain (loss) on hedges | 0 | (3) | 1 | (5)
SOUTHERN POWER CO | Cash Flow Hedging | Foreign currency derivatives | Interest Expense |  |  |  |
Derivative [Line Items] |  |  |  |
Gain (loss) on hedges | (2) | (3) | (8) | (9)
SOUTHERN POWER CO | Cash Flow Hedging | Foreign currency derivatives | Other Income (Expense) |  |  |  |
Derivative [Line Items] |  |  |  |
Gain (loss) on hedges | 1 | 24 | 68 | 7
SOUTHERN Co GAS | Cash Flow Hedging | Energy-related derivatives | Operating Expense |  |  |  |
Derivative [Line Items] |  |  |  |
Gain (loss) on hedges | 0 | 0 | 0 | (1)
SOUTHERN Co GAS | Cash Flow Hedging | Energy-related derivatives | Cost of Sales | Natural gas |  |  |  |
Derivative [Line Items] |  |  |  |
Gain (loss) on hedges | (1) | (4) | (1) | (34)
SOUTHERN Co GAS | Cash Flow Hedging | Interest rate derivatives | Interest Expense |  |  |  |
Derivative [Line Items] |  |  |  |
Gain (loss) on hedges | 0 | 0 | (1) | 0
SOUTHERN Co GAS | Fair Value Hedging | Interest rate derivatives | Interest Expense |  |  |  |
Derivative [Line Items] |  |  |  |
Gain (loss) on hedges | $ 7 | $ 28 | $ 28 | $ 18

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- Definition Net gain (loss) relating to components of the gain (loss) on the fair value hedging instrument excluded from the assessment of fair value hedge effectiveness. Recognized in earnings. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4A -Subparagraph (b)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480434/815-10-50-4A Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 815 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 4C -Subparagraph (d) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480434/815-10-50-4C Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 220 -SubTopic 40 -Name Accounting Standards Codification -Section 50 -Paragraph 21 -Subparagraph (i) -Publisher FASB -URI https://asc.fasb.org/1943274/2147476148/220-40-50-21 + Details
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