CONTINGENCIES, COMMITMENTS, AND GUARANTEES - GPC and GAS Environmental Remediation (Details) $ in Thousands		12 Months Ended
	Dec. 23, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($) state site	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Accrual for Environmental Loss Contingencies, Balance Sheet Classification [Abstract]
Accrued environmental remediation			$ 234,000	$ 268,000
Georgia Power
Environmental Exit Cost [Line Items]
Costs recovered annually			2,000	2,000	$ 2,000
Accrual for Environmental Loss Contingencies, Balance Sheet Classification [Abstract]
Environmental remediation liability, current			15,000	23,000
Accrued environmental remediation			0	0
Under recovered environmental remediation costs, other regulatory assets, current			12,000	2,000
Under recovered environmental remediation costs, other regulatory assets, deferred			40,000	53,000
Southern Company Gas
Accrual for Environmental Loss Contingencies, Balance Sheet Classification [Abstract]
Environmental remediation liability, current			36,000	26,000
Accrued environmental remediation			233,000	268,000
Under recovered environmental remediation costs, other regulatory assets, current			37,000	19,000
Under recovered environmental remediation costs, other regulatory assets, deferred			$ 260,000	292,000
Southern Company Gas | Manufactured Gas Plants
Environmental Exit Cost [Line Items]
Number of sites | site			40
Number of states in which subject to environmental remediation liabilities | state			4
Southern Company
Accrual for Environmental Loss Contingencies, Balance Sheet Classification [Abstract]
Environmental remediation liability, current			$ 51,000	49,000
Accrued environmental remediation			234,000	268,000
Under recovered environmental remediation costs, other regulatory assets, current			49,000	21,000
Under recovered environmental remediation costs, other regulatory assets, deferred			$ 300,000	$ 345,000
Scenario, Forecast | Georgia Power
Environmental Exit Cost [Line Items]
Costs recovered annually		$ 12,000
Mississippi Commission On Environmental Quality | Mississippi Power
Environmental Exit Cost [Line Items]
Civil penalties	$ 200