Retirement Benefits - Actuarial Assumptions (Details)	6 Months Ended		12 Months Ended
	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs			7.92%	8.16%	8.20%
Annual salary increase on net periodic benefit costs			4.37%	4.37%	3.59%
Discount rate, benefit obligation	4.40%		3.80%	4.40%
Annual salary increase, benefit obligation	4.37%		4.32%	4.37%
Other postretirement benefit plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs			6.84%	6.66%	6.97%
Annual salary increase on net periodic benefit costs			4.37%	4.37%	3.59%
Discount rate, benefit obligation	4.23%		3.68%	4.23%
Annual salary increase, benefit obligation	4.37%		4.32%	4.37%
Employee benefit obligations | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.40%	4.58%	4.17%
Employee benefit obligations | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.23%	4.38%	4.04%
Interest costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			3.77%	3.88%	4.17%
Interest costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			3.54%	3.66%	4.04%
Service costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.81%	4.98%	4.48%
Service costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.64%	4.85%	4.39%
MISSISSIPPI POWER CO | Pension plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs			7.95%	8.20%	8.20%
Annual salary increase on net periodic benefit costs			4.46%	4.46%	3.59%
Discount rate, benefit obligation	4.44%		3.80%	4.44%
Annual salary increase, benefit obligation	4.46%		4.46%	4.46%
MISSISSIPPI POWER CO | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs			6.88%	7.07%	7.23%
Annual salary increase on net periodic benefit costs			4.46%	4.46%	3.59%
Discount rate, benefit obligation	4.22%		3.68%	4.22%
Annual salary increase, benefit obligation	4.46%		4.46%	4.46%
MISSISSIPPI POWER CO | Employee benefit obligations | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.44%	4.69%	4.17%
MISSISSIPPI POWER CO | Employee benefit obligations | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.22%	4.47%	4.03%
MISSISSIPPI POWER CO | Interest costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			3.81%	3.97%	4.17%
MISSISSIPPI POWER CO | Interest costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			3.55%	3.66%	4.03%
MISSISSIPPI POWER CO | Service costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.83%	5.04%	4.49%
MISSISSIPPI POWER CO | Service costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.65%	4.88%	4.38%
ALABAMA POWER CO | Pension plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs			7.95%	8.20%	8.20%
Annual salary increase on net periodic benefit costs			4.46%	4.46%	3.59%
Discount rate, benefit obligation	4.44%		3.81%	4.44%
Annual salary increase, benefit obligation	4.46%		4.46%	4.46%
ALABAMA POWER CO | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs			6.83%	6.83%	7.17%
Annual salary increase on net periodic benefit costs			4.46%	4.46%	3.59%
Discount rate, benefit obligation	4.27%		3.71%	4.27%
Annual salary increase, benefit obligation	4.46%		4.46%	4.46%
ALABAMA POWER CO | Employee benefit obligations | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.44%	4.67%	4.18%
ALABAMA POWER CO | Employee benefit obligations | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.27%	4.51%	4.04%
ALABAMA POWER CO | Interest costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			3.76%	3.90%	4.18%
ALABAMA POWER CO | Interest costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			3.58%	3.69%	4.04%
ALABAMA POWER CO | Service costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.85%	5.07%	4.49%
ALABAMA POWER CO | Service costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.70%	4.96%	4.40%
GEORGIA POWER CO | Pension plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs			7.95%	8.20%	8.20%
Annual salary increase on net periodic benefit costs			4.46%	4.46%	3.59%
Discount rate, benefit obligation	4.40%		3.79%	4.40%
Annual salary increase, benefit obligation	4.46%		4.46%	4.46%
GEORGIA POWER CO | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs			6.79%	6.27%	6.48%
Annual salary increase on net periodic benefit costs			4.46%	4.46%	3.59%
Discount rate, benefit obligation	4.23%		3.68%	4.23%
Annual salary increase, benefit obligation	4.46%		4.46%	4.46%
GEORGIA POWER CO | Employee benefit obligations | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.40%	4.65%	4.18%
GEORGIA POWER CO | Employee benefit obligations | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.23%	4.49%	4.03%
GEORGIA POWER CO | Interest costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			3.72%	3.86%	4.18%
GEORGIA POWER CO | Interest costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			3.55%	3.67%	4.03%
GEORGIA POWER CO | Service costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.83%	5.03%	4.49%
GEORGIA POWER CO | Service costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.63%	4.88%	4.39%
GULF POWER CO | Pension plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs			7.95%	8.20%	8.20%
Annual salary increase on net periodic benefit costs			4.46%	4.46%	3.59%
Discount rate, benefit obligation	4.46%		3.82%	4.46%
Annual salary increase, benefit obligation	4.46%		4.46%	4.46%
GULF POWER CO | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs			7.81%	8.05%	8.07%
Annual salary increase on net periodic benefit costs			4.46%	4.46%	3.59%
Discount rate, benefit obligation	4.25%		3.69%	4.25%
Annual salary increase, benefit obligation	4.46%		4.46%	4.46%
GULF POWER CO | Employee benefit obligations
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs					4.18%
GULF POWER CO | Employee benefit obligations | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.46%	4.71%
GULF POWER CO | Employee benefit obligations | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.25%	4.51%	4.04%
GULF POWER CO | Interest costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			3.82%	3.97%	4.18%
GULF POWER CO | Interest costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			3.56%	3.68%	4.04%
GULF POWER CO | Service costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.81%	5.04%	4.48%
GULF POWER CO | Service costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.62%	4.88%	4.38%
SOUTHERN POWER CO | Employee benefit obligations | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			3.94%
Annual salary increase on net periodic benefit costs			4.46%
SOUTHERN POWER CO | Employee benefit obligations | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			3.81%
Annual salary increase on net periodic benefit costs			4.46%
SOUTHERN Co GAS
Pension Plans and Postretirement Plans
Pension band increase, benefit obligation	2.00%		2.00%	2.00%
SOUTHERN Co GAS | Pension plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs	7.75%		7.60%
Annual salary increase on net periodic benefit costs	3.50%		3.50%
Pension bad increase on net periodic benefit costs	2.00%
Discount rate, benefit obligation	4.39%		3.74%	4.39%
Annual salary increase, benefit obligation	3.50%		2.88%	3.50%
Pension band increase, benefit obligation	2.00%			2.00%
SOUTHERN Co GAS | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs	5.93%		6.03%
Annual salary increase on net periodic benefit costs	3.50%		3.50%
Discount rate, benefit obligation	4.15%		3.62%	4.15%
Annual salary increase, benefit obligation	3.50%		2.56%	3.50%
SOUTHERN Co GAS | Interest costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs	3.21%		3.76%
SOUTHERN Co GAS | Interest costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs	2.84%		3.40%
SOUTHERN Co GAS | Service costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs	4.07%		4.64%
SOUTHERN Co GAS | Service costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs	3.96%		4.55%
SOUTHERN Co GAS | Predecessor | Pension plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs		7.80%			7.80%
Annual salary increase on net periodic benefit costs		3.70%			3.70%
Pension bad increase on net periodic benefit costs		2.00%			2.00%
SOUTHERN Co GAS | Predecessor | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs		6.60%			7.80%
Annual salary increase on net periodic benefit costs		3.70%			3.70%
SOUTHERN Co GAS | Predecessor | Interest costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs		4.00%			4.20%
SOUTHERN Co GAS | Predecessor | Interest costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs		3.60%			4.00%
SOUTHERN Co GAS | Predecessor | Service costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs		4.80%			4.20%
SOUTHERN Co GAS | Predecessor | Service costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs		4.70%			4.00%