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Summary of Significant Accounting Policies - Intangible Assets (Details) - Southern Power [Member]
$ in Millions
Dec. 31, 2015
USD ($)
Acquired Finite-Lived Intangible Assets [Line Items]  
2016 $ 10
2017 17
2018 17
2019 17
2020 17
2021 and beyond 239
Total $ 317