RETIREMENT BENEFITS - Narrative (Details)	1 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($) employee	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Plan asset amortization period		5 years
Southern Power
Defined Benefit Plan Disclosure [Line Items]
Prior service costs and net gains/losses in OCI	$ 36,000,000
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	13,808,000,000	$ 14,788,000,000	$ 12,763,000,000
Plan assets	$ 12,992,000,000	14,057,000,000	11,611,000,000
Actuarial gain (loss)		$ (2,346,000,000)	$ 1,143,000,000
Change in discount rate		1.08%	0.69%
Pension plans | Qualified pension plan
Defined Benefit Plan Disclosure [Line Items]
Expected future employer contributions, next fiscal year		$ 0
Projected benefit obligation		14,055,000,000
Pension plans | Southern Company Services, Inc.
Defined Benefit Plan Disclosure [Line Items]
Number of employees transferred | employee	538
Pension plans | Southern Power
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 139,000,000	185,000,000	$ 123,000,000
Plan assets	138,000,000	169,000,000	123,000,000
Actuarial gain (loss)		(54,000,000)	24,000,000
Pension plans | Southern Power | Qualified pension plan
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation		159,000,000
Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Expected future employer contributions, next fiscal year		0
Projected benefit obligation	2,339,000,000	1,985,000,000	1,865,000,000
Plan assets	1,053,000,000	1,061,000,000	928,000,000
Actuarial gain (loss)		(223,000,000)	432,000,000
Other postretirement benefit plans | Southern Power
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	11,000,000	11,000,000	9,000,000
Plan assets	$ 0	0	0
Actuarial gain (loss)		$ (2,000,000)	$ 2,000,000