Summary of Significant Accounting Policies - Schedule of Amortization of Other Intangible Assets (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Amortization associated with other intangible assets			$ 89	$ 124	$ 50
Estimated future amortization, 2019			61
Estimated future amortization, 2020			50
Estimated future amortization, 2021			43
Estimated future amortization, 2022			39
Estimated future amortization, 2023			38
Southern Power
Finite-Lived Intangible Assets [Line Items]
Amortization associated with other intangible assets			25	25	$ 10
Estimated future amortization, 2019			20
Estimated future amortization, 2020			20
Estimated future amortization, 2021			20
Estimated future amortization, 2022			20
Estimated future amortization, 2023			20
Southern Company Gas
Finite-Lived Intangible Assets [Line Items]
Estimated future amortization, 2019			29
Estimated future amortization, 2020			19
Estimated future amortization, 2021			13
Estimated future amortization, 2022			10
Estimated future amortization, 2023			9
Southern Company Gas | Wholesale gas services
Finite-Lived Intangible Assets [Line Items]
Amortization associated with other intangible assets	$ 2		20	32
Southern Company Gas | Wholesale gas services | Predecessor
Finite-Lived Intangible Assets [Line Items]
Amortization associated with other intangible assets		$ 0
Southern Company Gas | Gas marketing services
Finite-Lived Intangible Assets [Line Items]
Amortization associated with other intangible assets	$ 32		$ 32	$ 54
Southern Company Gas | Gas marketing services | Predecessor
Finite-Lived Intangible Assets [Line Items]
Amortization associated with other intangible assets		$ 8