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Commitments - Contractual Obligations - Pipeline Charges, Storage Capacity, and Gas Supply (Details) - Pipeline Charges, Storage Capacity, and Gas Supply
$ in Millions
Dec. 31, 2017
USD ($)
Long-term Purchase Commitment [Line Items]  
2018 $ 813
2019 552
2020 416
2021 375
2022 339
2023 and thereafter 2,294
Total 4,789
SOUTHERN Co GAS  
Long-term Purchase Commitment [Line Items]  
Total $ 4,789