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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Natural Gas for Sale (Narrative) (Details) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2021 | Dec. 31, 2020 | Dec. 31, 2019
Nicor Gas |  |  |
Inventory [Line Items] |  |  |
LIFO inventory balance | $ 166 |  |
Estimated replacement cost of inventory | 470 |  |
Southern Company Gas |  |  |
Inventory [Line Items] |  |  |
LOCOM adjustments | $ 1 | $ 1 | $ 21

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