Retirement Benefits - Actuarial Assumptions (Details)	6 Months Ended		12 Months Ended
	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2017
Pension plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs			8.16%	8.20%	8.20%
Annual salary increase on net periodic benefit costs			4.37%	3.59%	3.59%
Discount rate, benefit obligation	4.40%		4.40%	4.67%
Annual salary increase, benefit obligation	4.37%		4.37%	4.46%
Other postretirement benefit plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs			6.66%	6.97%	7.15%
Annual salary increase on net periodic benefit costs			4.37%	3.59%	3.59%
Discount rate, benefit obligation	4.23%		4.23%	4.51%
Annual salary increase, benefit obligation	4.37%		4.37%	4.46%
Employee benefit obligations | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.58%	4.17%	5.02%
Employee benefit obligations | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.38%	4.04%	4.85%
Interest costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			3.88%	4.17%	5.02%
Interest costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			3.66%	4.04%	4.85%
Service costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.98%	4.48%	5.02%
Service costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.85%	4.39%	4.85%
Southern Company Gas [Member]
Pension Plans and Postretirement Plans
Pension band increase, benefit obligation	2.00%		2.00%
Southern Company Gas [Member] | Scenario, Forecast [Member]
Pension Plans and Postretirement Plans
Pension band increase, benefit obligation						2.00%
Southern Company Gas [Member] | Successor [Member] | Pension plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs	7.75%
Annual salary increase on net periodic benefit costs	3.50%
Pension bad increase on net periodic benefit costs	2.00%
Discount rate, benefit obligation	4.39%		4.39%
Annual salary increase, benefit obligation	3.50%		3.50%
Pension band increase, benefit obligation	2.00%		2.00%
Southern Company Gas [Member] | Successor [Member] | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs	5.93%
Annual salary increase on net periodic benefit costs	3.50%
Discount rate, benefit obligation	4.15%		4.15%
Annual salary increase, benefit obligation	3.50%		3.50%
Southern Company Gas [Member] | Successor [Member] | Interest costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs	3.21%
Southern Company Gas [Member] | Successor [Member] | Interest costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs	2.84%
Southern Company Gas [Member] | Successor [Member] | Service costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs	4.07%
Southern Company Gas [Member] | Successor [Member] | Service costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs	3.96%
Southern Company Gas [Member] | Predecessor [Member] | Pension plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs		7.80%		7.80%	7.80%
Annual salary increase on net periodic benefit costs		3.70%		3.70%	3.70%
Pension bad increase on net periodic benefit costs		2.00%		2.00%	2.00%
Discount rate, benefit obligation				4.60%
Annual salary increase, benefit obligation				3.70%
Pension band increase, benefit obligation				2.00%
Southern Company Gas [Member] | Predecessor [Member] | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs		6.60%		7.80%	7.80%
Annual salary increase on net periodic benefit costs		3.70%		3.70%	3.70%
Discount rate, benefit obligation				4.40%
Annual salary increase, benefit obligation				3.70%
Southern Company Gas [Member] | Predecessor [Member] | Interest costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs		4.00%		4.20%	5.00%
Southern Company Gas [Member] | Predecessor [Member] | Interest costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs		3.60%		4.00%	4.70%
Southern Company Gas [Member] | Predecessor [Member] | Service costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs		4.80%		4.20%	5.00%
Southern Company Gas [Member] | Predecessor [Member] | Service costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs		4.70%		4.00%	4.70%
Alabama Power [Member] | Pension plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs			8.20%	8.20%	8.20%
Annual salary increase on net periodic benefit costs			4.46%	3.59%	3.59%
Discount rate, benefit obligation	4.44%		4.44%	4.67%
Annual salary increase, benefit obligation	4.46%		4.46%	4.46%
Alabama Power [Member] | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs			6.83%	7.17%	7.34%
Annual salary increase on net periodic benefit costs			4.46%	3.59%	3.59%
Discount rate, benefit obligation	4.27%		4.27%	4.51%
Annual salary increase, benefit obligation	4.46%		4.46%	4.46%
Alabama Power [Member] | Employee benefit obligations | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.67%	4.18%	5.02%
Alabama Power [Member] | Employee benefit obligations | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.51%	4.04%	4.86%
Alabama Power [Member] | Interest costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			3.90%	4.18%	5.02%
Alabama Power [Member] | Interest costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			3.69%	4.04%	4.86%
Alabama Power [Member] | Service costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			5.07%	4.49%	5.02%
Alabama Power [Member] | Service costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.96%	4.40%	4.86%
Georgia Power [Member] | Pension plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs			8.20%	8.20%	8.20%
Annual salary increase on net periodic benefit costs			4.46%	3.59%	3.59%
Discount rate, benefit obligation	4.40%		4.40%	4.65%
Annual salary increase, benefit obligation	4.46%		4.46%	4.46%
Georgia Power [Member] | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs			6.27%	6.48%	6.75%
Annual salary increase on net periodic benefit costs			4.46%	3.59%	3.59%
Discount rate, benefit obligation	4.23%		4.23%	4.49%
Annual salary increase, benefit obligation	4.46%		4.46%	4.46%
Georgia Power [Member] | Employee benefit obligations | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.65%	4.18%	5.02%
Georgia Power [Member] | Employee benefit obligations | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.49%	4.03%	4.85%
Georgia Power [Member] | Interest costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			3.86%	4.18%	5.02%
Georgia Power [Member] | Interest costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			3.67%	4.03%	4.85%
Georgia Power [Member] | Service costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			5.03%	4.49%	5.02%
Georgia Power [Member] | Service costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.88%	4.39%	4.85%
Gulf Power [Member] | Pension plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs			8.20%	8.20%	8.20%
Annual salary increase on net periodic benefit costs			4.46%	3.59%	3.59%
Discount rate, benefit obligation	4.46%		4.46%	4.71%
Annual salary increase, benefit obligation	4.46%		4.46%	4.46%
Gulf Power [Member] | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs			8.05%	8.07%	8.08%
Annual salary increase on net periodic benefit costs			4.46%	3.59%	3.59%
Discount rate, benefit obligation	4.25%		4.25%	4.51%
Annual salary increase, benefit obligation	4.46%		4.46%	4.46%
Gulf Power [Member] | Employee benefit obligations | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.71%	4.18%	5.02%
Gulf Power [Member] | Employee benefit obligations | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.51%	4.04%	4.86%
Gulf Power [Member] | Interest costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			3.97%	4.18%	5.02%
Gulf Power [Member] | Interest costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			3.68%	4.04%	4.86%
Gulf Power [Member] | Service costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			5.04%	4.48%	5.02%
Gulf Power [Member] | Service costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.88%	4.38%	4.86%
Mississippi Power [Member] | Pension plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs			8.20%	8.20%	8.20%
Annual salary increase on net periodic benefit costs			4.46%	3.59%	3.59%
Discount rate, benefit obligation	4.44%		4.44%	4.69%
Annual salary increase, benefit obligation	4.46%		4.46%	4.46%
Mississippi Power [Member] | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Long-term return on plan assets on net periodic benefit costs			7.07%	7.23%	7.30%
Annual salary increase on net periodic benefit costs			4.46%	3.59%	3.59%
Discount rate, benefit obligation	4.22%		4.22%	4.47%
Annual salary increase, benefit obligation	4.46%		4.46%	4.46%
Mississippi Power [Member] | Employee benefit obligations | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.69%	4.17%	5.01%
Mississippi Power [Member] | Employee benefit obligations | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.47%	4.03%	4.85%
Mississippi Power [Member] | Interest costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			3.97%	4.17%	5.01%
Mississippi Power [Member] | Interest costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			3.66%	4.03%	4.85%
Mississippi Power [Member] | Service costs | Pension plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			5.04%	4.49%	5.01%
Mississippi Power [Member] | Service costs | Other postretirement benefit plans
Pension Plans and Postretirement Plans
Discount rate on net periodic benefit costs			4.88%	4.38%	4.85%