RETIREMENT BENEFITS - Actuarial Assumptions (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	4.49%	3.80%	4.40%
Expected long-term return on plan assets	7.75%	7.95%	7.92%
Annual salary increase, net periodic costs	4.34%	4.34%	4.37%
Pension plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	4.12%	3.45%	3.77%
Pension plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	4.70%	3.98%	4.81%
Pension plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	3.41%	4.49%
Annual salary increase	4.73%	4.34%
Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	4.37%	3.68%	4.23%
Expected long-term return on plan assets	6.86%	6.83%	6.84%
Annual salary increase, net periodic costs	4.34%	4.34%	4.37%
Other postretirement benefit plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	3.98%	3.29%	3.54%
Other postretirement benefit plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	4.63%	3.91%	4.64%
Other postretirement benefit plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	3.24%	4.37%
Annual salary increase	4.73%	4.34%
Alabama Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	4.51%	3.81%	4.44%
Expected long-term return on plan assets	7.75%	7.95%	7.95%
Annual salary increase, net periodic costs	4.46%	4.46%	4.46%
Alabama Power | Pension plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	4.14%	3.45%	3.76%
Alabama Power | Pension plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	4.73%	4.00%	4.85%
Alabama Power | Pension plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	3.44%	4.51%
Annual salary increase	4.73%	4.46%
Alabama Power | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	4.40%	3.71%	4.27%
Expected long-term return on plan assets	6.76%	6.83%	6.83%
Annual salary increase, net periodic costs	4.46%	4.46%	4.46%
Alabama Power | Other postretirement benefit plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	4.01%	3.31%	3.58%
Alabama Power | Other postretirement benefit plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	4.67%	3.93%	4.70%
Alabama Power | Other postretirement benefit plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	3.28%	4.40%
Annual salary increase	4.73%	4.46%
Georgia Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	4.48%	3.79%	4.40%
Expected long-term return on plan assets	7.75%	7.95%	7.95%
Annual salary increase, net periodic costs	4.46%	4.46%	4.46%
Georgia Power | Pension plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	4.10%	3.42%	3.72%
Georgia Power | Pension plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	4.72%	3.99%	4.83%
Georgia Power | Pension plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	3.40%	4.48%
Annual salary increase	4.73%	4.46%
Georgia Power | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	4.36%	3.68%	4.23%
Expected long-term return on plan assets	6.85%	6.80%	6.79%
Annual salary increase, net periodic costs	4.46%	4.46%	4.46%
Georgia Power | Other postretirement benefit plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	3.97%	3.29%	3.55%
Georgia Power | Other postretirement benefit plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	4.64%	3.91%	4.63%
Georgia Power | Other postretirement benefit plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	3.22%	4.36%
Annual salary increase	4.73%	4.46%
Mississippi Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	4.49%	3.80%	4.44%
Expected long-term return on plan assets	7.75%	7.95%	7.95%
Annual salary increase, net periodic costs	4.46%	4.46%	4.46%
Mississippi Power | Pension plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	4.12%	3.46%	3.81%
Mississippi Power | Pension plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	4.73%	3.99%	4.83%
Mississippi Power | Pension plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	3.41%	4.49%
Annual salary increase	4.73%	4.46%
Mississippi Power | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	4.35%	3.68%	4.22%
Expected long-term return on plan assets	6.79%	6.99%	6.88%
Annual salary increase, net periodic costs	4.46%	4.46%	4.46%
Mississippi Power | Other postretirement benefit plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	3.95%	3.29%	3.55%
Mississippi Power | Other postretirement benefit plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	4.64%	3.91%	4.65%
Mississippi Power | Other postretirement benefit plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	3.22%	4.35%
Annual salary increase	4.73%	4.46%
Southern Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	4.65%	3.94%
Expected long-term return on plan assets	7.75%	7.95%
Annual salary increase, net periodic costs	4.46%	4.46%
Southern Power | Pension plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	4.35%	3.69%
Southern Power | Pension plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	4.75%	4.01%
Southern Power | Pension plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	3.52%	4.65%
Annual salary increase	4.73%	4.46%
Southern Power | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	4.50%	3.81%
Expected long-term return on plan assets	0.00%	0.00%
Annual salary increase, net periodic costs	4.46%	4.46%
Southern Power | Other postretirement benefit plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	4.14%	3.47%
Southern Power | Other postretirement benefit plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	4.65%	3.93%
Southern Power | Other postretirement benefit plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	3.39%	4.50%
Annual salary increase	4.73%	4.46%
Southern Company Gas | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	4.47%	3.74%	4.39%
Expected long-term return on plan assets	7.75%	7.95%	7.60%
Annual salary increase, net periodic costs	3.07%	3.07%	3.50%
Southern Company Gas | Pension plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	4.11%	3.41%	3.76%
Southern Company Gas | Pension plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	4.57%	3.84%	4.64%
Southern Company Gas | Pension plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	3.39%	4.47%
Annual salary increase	4.73%	3.07%
Southern Company Gas | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	4.32%	3.62%	4.15%
Expected long-term return on plan assets	6.49%	5.89%	6.03%
Annual salary increase, net periodic costs	3.07%	3.07%	3.50%
Southern Company Gas | Other postretirement benefit plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	3.91%	3.21%	3.40%
Southern Company Gas | Other postretirement benefit plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	4.56%	3.82%	4.55%
Southern Company Gas | Other postretirement benefit plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	3.19%	4.32%
Annual salary increase	4.73%	3.07%