Retirement Benefits - Actuarial Assumptions (Details)	6 Months Ended		12 Months Ended
	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	4.58%		3.80%	4.40%	4.58%
Expected long-term return on plan assets			7.95%	7.92%	8.16%
Annual salary increase, net periodic costs			4.34%	4.37%	4.37%
Pension plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs			3.45%	3.77%	3.88%
Pension plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs			3.98%	4.81%	4.98%
Pension plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations			4.49%	3.80%
Annual salary increase			4.34%	4.32%
Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	4.38%		3.68%	4.23%	4.38%
Expected long-term return on plan assets			6.83%	6.84%	6.66%
Annual salary increase, net periodic costs			4.34%	4.37%	4.37%
Other postretirement benefit plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs			3.29%	3.54%	3.66%
Other postretirement benefit plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs			3.91%	4.64%	4.85%
Other postretirement benefit plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations			4.37%	3.68%
Annual salary increase			4.34%	4.32%
Alabama Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	4.67%		3.81%	4.44%	4.67%
Expected long-term return on plan assets			7.95%	7.95%	8.20%
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			3.45%	3.76%	3.90%
Alabama Power | Pension plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs			4.00%	4.85%	5.07%
Alabama Power | Pension plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations			4.51%	3.81%
Annual salary increase			4.46%	4.46%
Alabama Power | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	4.51%		3.71%	4.27%	4.51%
Expected long-term return on plan assets			6.83%	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			3.31%	3.58%	3.69%
Alabama Power | Other postretirement benefit plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs			3.93%	4.70%	4.96%
Alabama Power | Other postretirement benefit plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations			4.40%	3.71%
Annual salary increase			4.46%	4.46%
Georgia Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	4.65%		3.79%	4.40%	4.65%
Expected long-term return on plan assets			7.95%	7.95%	8.20%
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			3.42%	3.72%	3.86%
Georgia Power | Pension plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs			3.99%	4.83%	5.03%
Georgia Power | Pension plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations			4.48%	3.79%
Annual salary increase			4.46%	4.46%
Georgia Power | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	4.49%		3.68%	4.23%	4.49%
Expected long-term return on plan assets			6.80%	6.79%	6.27%
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			3.29%	3.55%	3.67%
Georgia Power | Other postretirement benefit plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs			3.91%	4.63%	4.88%
Georgia Power | Other postretirement benefit plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations			4.36%	3.68%
Annual salary increase			4.46%	4.46%
Mississippi Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	4.69%		3.80%	4.44%	4.69%
Expected long-term return on plan assets			7.95%	7.95%	8.20%
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			3.46%	3.81%	3.97%
Mississippi Power | Pension plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs			3.99%	4.83%	5.04%
Mississippi Power | Pension plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations			4.49%	3.80%
Annual salary increase			4.46%	4.46%
Mississippi Power | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations	4.47%		3.68%	4.22%	4.47%
Expected long-term return on plan assets			6.99%	6.88%	7.07%
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			3.29%	3.55%	3.66%
Mississippi Power | Other postretirement benefit plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs			3.91%	4.65%	4.88%
Mississippi Power | Other postretirement benefit plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations			4.35%	3.68%
Annual salary increase			4.46%	4.46%
Southern Power | Pension plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations			3.94%
Expected long-term return on plan assets			7.95%
Annual salary increase, net periodic costs			4.46%
Southern Power | Pension plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs			3.69%
Southern Power | Pension plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs			4.01%
Southern Power | Pension plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations			4.65%	3.94%
Annual salary increase			4.46%	4.46%
Southern Power | Other postretirement benefit plans
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations			3.81%
Expected long-term return on plan assets			0.00%
Annual salary increase, net periodic costs			4.46%
Southern Power | Other postretirement benefit plans | Discount rate – interest costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs			3.47%
Southern Power | Other postretirement benefit plans | Discount rate – service costs
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs			3.93%
Southern Power | Other postretirement benefit plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations			4.50%	3.81%
Annual salary increase			4.46%	4.46%
Southern Company Gas | Pension plans | Successor
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	3.85%		3.74%	4.39%
Expected long-term return on plan assets	7.75%		7.95%	7.60%
Annual salary increase, net periodic costs	3.50%		3.07%	3.50%
Pension band increase	2.00%
Southern Company Gas | Pension plans | Predecessor
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs		4.60%
Expected long-term return on plan assets		7.80%
Annual salary increase, net periodic costs		3.70%
Pension band increase		2.00%
Southern Company Gas | Pension plans | Discount rate – interest costs | Successor
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	3.21%		3.41%	3.76%
Southern Company Gas | Pension plans | Discount rate – interest costs | Predecessor
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs		4.00%
Southern Company Gas | Pension plans | Discount rate – service costs | Successor
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	4.07%		3.84%	4.64%
Southern Company Gas | Pension plans | Discount rate – service costs | Predecessor
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs		4.80%
Southern Company Gas | Pension plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations			4.47%	3.74%
Annual salary increase			3.07%	2.88%
Southern Company Gas | Other postretirement benefit plans | Successor
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	3.61%		3.62%	4.15%
Expected long-term return on plan assets	5.93%		5.89%	6.03%
Annual salary increase, net periodic costs	3.50%		3.07%	3.50%
Southern Company Gas | Other postretirement benefit plans | Predecessor
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs		4.40%
Expected long-term return on plan assets		6.60%
Annual salary increase, net periodic costs		3.70%
Southern Company Gas | Other postretirement benefit plans | Discount rate – interest costs | Successor
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	2.84%		3.21%	3.40%
Southern Company Gas | Other postretirement benefit plans | Discount rate – interest costs | Predecessor
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs		3.60%
Southern Company Gas | Other postretirement benefit plans | Discount rate – service costs | Successor
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs	3.96%		3.82%	4.55%
Southern Company Gas | Other postretirement benefit plans | Discount rate – service costs | Predecessor
Defined Benefit Plan Disclosure [Line Items]
Discount rate, net periodic benefit costs		4.70%
Southern Company Gas | Other postretirement benefit plans | Employee benefit obligations
Defined Benefit Plan Disclosure [Line Items]
Discount rate – benefit obligations			4.32%	3.62%
Annual salary increase			3.07%	2.56%