SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Schedule of Amortization of Other Intangible Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Amortization associated with other intangible assets	$ 61	$ 89	$ 124
Estimated future amortization, 2020	48
Estimated future amortization, 2021	42
Estimated future amortization, 2022	38
Estimated future amortization, 2023	37
Estimated future amortization, 2024	35
Southern Power
Finite-Lived Intangible Assets [Line Items]
Amortization associated with other intangible assets	19	25	25
Estimated future amortization, 2020	20
Estimated future amortization, 2021	20
Estimated future amortization, 2022	20
Estimated future amortization, 2023	20
Estimated future amortization, 2024	20
Southern Company Gas
Finite-Lived Intangible Assets [Line Items]
Amortization associated with other intangible assets	31	52	86
Estimated future amortization, 2020	19
Estimated future amortization, 2021	13
Estimated future amortization, 2022	10
Estimated future amortization, 2023	9
Estimated future amortization, 2024	7
Southern Company Gas | Gas marketing services
Finite-Lived Intangible Assets [Line Items]
Amortization associated with other intangible assets	23	32	54
Southern Company Gas | Wholesale gas services
Finite-Lived Intangible Assets [Line Items]
Amortization associated with other intangible assets	8	20	32
Operating Revenues
Finite-Lived Intangible Assets [Line Items]
Amortization associated with other intangible assets	$ 27	$ 45	$ 57