Retirement Benefits - Narrative (Details)	1 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($) employee	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
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		$ 36,000,000
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Employer contributions		$ 55,000,000	52,000,000
Projected benefit obligation	13,808,000,000	12,763,000,000	13,808,000,000	$ 12,385,000,000
Plan assets	$ 12,992,000,000	11,611,000,000	12,992,000,000	11,583,000,000
Actuarial gain (loss)		$ 1,143,000,000	$ (1,297,000,000)
Change in discount rate		0.69%	(0.60%)
Pension plans | Qualified Plan
Defined Benefit Plan Disclosure [Line Items]
Employer contributions		$ 0
Expected future employer contributions, next fiscal year		0
Projected benefit obligation		12,135,000,000
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]
Employer contributions		0	$ 0
Projected benefit obligation	$ 139,000,000	123,000,000	139,000,000	0
Plan assets	138,000,000	123,000,000	138,000,000	0
Actuarial gain (loss)		24,000,000	0
Pension plans | Southern Power | Qualified Plan
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation		122,000,000
Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Employer contributions		73,000,000	84,000,000
Expected future employer contributions, next fiscal year		0
Projected benefit obligation	2,339,000,000	1,865,000,000	2,339,000,000	2,297,000,000
Plan assets	1,053,000,000	928,000,000	1,053,000,000	944,000,000
Actuarial gain (loss)		432,000,000	(65,000,000)
Other postretirement benefit plans | Southern Power
Defined Benefit Plan Disclosure [Line Items]
Employer contributions		1,000,000	0
Projected benefit obligation	11,000,000	9,000,000	11,000,000	0
Plan assets	$ 0	0	0	$ 0
Actuarial gain (loss)		$ 2,000,000	$ 0