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Commitments - Contractual Obligations - Pipeline Charges, Storage Capacity, and Gas Supply (Details) - Pipeline Charges, Storage Capacity, and Gas Supply - Southern Company Gas
$ in Millions
Dec. 31, 2018
USD ($)
Long-term Purchase Commitment [Line Items]  
2019 $ 781
2020 584
2021 520
2022 489
2023 412
2024 and thereafter 1,871
Total $ 4,657