RETIREMENT BENEFITS - Actuarial Assumptions (Details)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	2.81%	3.41%	4.49%
Expected long-term return on plan assets	8.25%	8.25%	7.75%
Annual salary increase, net periodic costs	4.80%	4.73%	4.34%
Pension plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	2.13%	2.99%	4.12%
Pension plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	3.18%	3.66%	4.70%
Pension plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	3.09%	2.81%
Annual salary increase	4.80%	4.80%
Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	2.56%	3.24%	4.37%
Expected long-term return on plan assets	7.09%	7.25%	6.86%
Annual salary increase, net periodic costs	4.80%	4.73%	4.34%
Other postretirement benefit plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	1.84%	2.80%	3.98%
Other postretirement benefit plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	3.07%	3.57%	4.63%
Other postretirement benefit plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	2.90%	2.56%
Annual salary increase	4.80%	4.80%
Alabama Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	2.85%	3.44%	4.51%
Expected long-term return on plan assets	8.25%	8.25%	7.75%
Annual salary increase, net periodic costs	4.80%	4.73%	4.46%
Alabama Power | Pension plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	2.17%	3.01%	4.14%
Alabama Power | Pension plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	3.23%	3.69%	4.73%
Alabama Power | Pension plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	3.12%	2.85%
Annual salary increase	4.80%	4.80%
Alabama Power | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	2.63%	3.28%	4.40%
Expected long-term return on plan assets	7.18%	7.36%	6.76%
Annual salary increase, net periodic costs	4.80%	4.73%	4.46%
Alabama Power | Other postretirement benefit plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	1.91%	2.84%	4.01%
Alabama Power | Other postretirement benefit plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	3.13%	3.61%	4.67%
Alabama Power | Other postretirement benefit plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	2.95%	2.63%
Annual salary increase	4.80%	4.80%
Georgia Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	2.79%	3.40%	4.48%
Expected long-term return on plan assets	8.25%	8.25%	7.75%
Annual salary increase, net periodic costs	4.80%	4.73%	4.46%
Georgia Power | Pension plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	2.09%	2.96%	4.10%
Georgia Power | Pension plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	3.21%	3.67%	4.72%
Georgia Power | Pension plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	3.07%	2.79%
Annual salary increase	4.80%	4.80%
Georgia Power | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	2.52%	3.22%	4.36%
Expected long-term return on plan assets	6.84%	7.05%	6.85%
Annual salary increase, net periodic costs	4.80%	4.73%	4.46%
Georgia Power | Other postretirement benefit plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	1.82%	2.79%	3.97%
Georgia Power | Other postretirement benefit plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	3.08%	3.57%	4.64%
Georgia Power | Other postretirement benefit plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	2.87%	2.52%
Annual salary increase	4.80%	4.80%
Mississippi Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	2.80%	3.41%	4.49%
Expected long-term return on plan assets	8.25%	8.25%	7.75%
Annual salary increase, net periodic costs	4.80%	4.73%	4.46%
Mississippi Power | Pension plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	2.12%	2.99%	4.12%
Mississippi Power | Pension plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	3.20%	3.67%	4.73%
Mississippi Power | Pension plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	3.07%	2.80%
Annual salary increase	4.80%	4.80%
Mississippi Power | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	2.53%	3.22%	4.35%
Expected long-term return on plan assets	6.98%	7.07%	6.79%
Annual salary increase, net periodic costs	4.80%	4.73%	4.46%
Mississippi Power | Other postretirement benefit plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	1.78%	2.76%	3.95%
Mississippi Power | Other postretirement benefit plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	3.06%	3.57%	4.64%
Mississippi Power | Other postretirement benefit plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	2.88%	2.53%
Annual salary increase	4.80%	4.80%
Southern Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	2.99%	3.52%	4.65%
Expected long-term return on plan assets	8.25%	8.25%	7.75%
Annual salary increase, net periodic costs	4.80%	4.73%	4.46%
Southern Power | Pension plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	2.46%	3.18%	4.35%
Southern Power | Pension plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	3.22%	3.70%	4.75%
Southern Power | Pension plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	3.21%	2.99%
Annual salary increase	4.80%	4.80%
Southern Power | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	2.78%	3.39%	4.50%
Expected long-term return on plan assets	0.00%	0.00%	0.00%
Annual salary increase, net periodic costs	4.80%	4.73%	4.46%
Southern Power | Other postretirement benefit plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	2.12%	2.97%	4.14%
Southern Power | Other postretirement benefit plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	3.05%	3.57%	4.65%
Southern Power | Other postretirement benefit plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	3.07%	2.78%
Annual salary increase	4.80%	4.80%
Southern Company Gas | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	2.75%	3.39%	4.47%
Expected long-term return on plan assets	8.25%	8.25%	7.75%
Annual salary increase, net periodic costs	4.80%	4.73%	3.07%
Southern Company Gas | Pension plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	2.10%	2.99%	4.11%
Southern Company Gas | Pension plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	2.97%	3.53%	4.57%
Southern Company Gas | Pension plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	3.04%	2.75%
Annual salary increase	4.80%	4.80%
Southern Company Gas | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	2.46%	3.19%	4.32%
Expected long-term return on plan assets	6.54%	6.69%	6.49%
Annual salary increase, net periodic costs	4.80%	4.73%	3.07%
Southern Company Gas | Other postretirement benefit plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	1.64%	2.71%	3.91%
Southern Company Gas | Other postretirement benefit plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	3.01%	3.52%	4.56%
Southern Company Gas | Other postretirement benefit plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	2.82%	2.46%
Annual salary increase	4.80%	4.80%