Contingencies - GPC and GAS Environmental Remediation (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) state site	Dec. 31, 2017 USD ($)
Accrual for Environmental Loss Contingencies, Balance Sheet Classification [Abstract]
Accrued environmental remediation	$ 268	$ 389
Georgia Power
Environmental Exit Cost [Line Items]
Costs recovered annually	2
Accrual for Environmental Loss Contingencies, Balance Sheet Classification [Abstract]
Environmental remediation liability, current	23	22
Accrued environmental remediation	0	0
Under recovered environmental remediation costs, other regulatory assets, current	2	2
Under recovered environmental remediation costs, other regulatory assets, deferred	53	47
Southern Company Gas
Accrual for Environmental Loss Contingencies, Balance Sheet Classification [Abstract]
Environmental remediation liability, current	26	46
Accrued environmental remediation	268	342
Under recovered environmental remediation costs, other regulatory assets, current	19	31
Under recovered environmental remediation costs, other regulatory assets, deferred	$ 292	379
Southern Company Gas | Manufactured Gas Plants
Environmental Exit Cost [Line Items]
# of sites | site	40
Number of states in which subject to environmental remediation liabilities | state	4
Southern Company Gas | Location One
Environmental Exit Cost [Line Items]
Environmental remediation liabilities of former sites	$ 2
Southern Company Gas | Location One | Elizabethtown Gas
Environmental Exit Cost [Line Items]
Environmental remediation liabilities of former sites		85
Southern Company
Accrual for Environmental Loss Contingencies, Balance Sheet Classification [Abstract]
Environmental remediation liability, current	49	73
Accrued environmental remediation	268	389
Under recovered environmental remediation costs, other regulatory assets, current	21	38
Under recovered environmental remediation costs, other regulatory assets, deferred	$ 345	$ 473