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Commitments - Contractual Obligations - Pipeline Charges, Storage Capacity, and Gas Supply (Details) - Pipeline Charges Storage Capacity And Gas Supply [Member]
$ in Millions
Dec. 31, 2016
USD ($)
Long-term Purchase Commitment [Line Items]  
2017 $ 822
2018 602
2019 447
2020 394
2021 352
2022 and thereafter 2,591
Total 5,208
Southern Company Gas [Member]  
Long-term Purchase Commitment [Line Items]  
Total $ 5,208