Summary of Significant Accounting Policies - AFUC and Interest Capitalized (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Line Items]
Allowance for equity funds used during construction			$ 160	$ 202	$ 226
AFUDC, net of income taxes			25.50%	11.40%	12.80%
Interest, net of amounts capitalized			$ 1,700	$ 1,100	$ 809
Net cash paid for capitalized interest			89	125	124
ALABAMA POWER CO
Accounting Policies [Line Items]
Allowance for equity funds used during construction			$ 39	$ 28	$ 60
Composite rate used for allowance for funds used during construction	8.40%		8.30%	8.40%	8.70%
AFUDC, net of income taxes			5.70%	4.20%	9.30%
Interest, net of amounts capitalized			$ 285	$ 277	$ 250
Net cash paid for capitalized interest			$ 15	$ 11	22
GEORGIA POWER CO
Accounting Policies [Line Items]
Composite rate used for allowance for funds used during construction	6.90%		5.60%	6.90%		6.50%
Allowance for funds used during construction, capitalized interest			$ 63	$ 68	$ 56
AFUDC, net of income taxes			3.80%	4.60%	3.90%
Interest, net of amounts capitalized			$ 386	$ 375	$ 353
Net cash paid for capitalized interest			$ 23	$ 20	$ 16
GULF POWER CO
Accounting Policies [Line Items]
Composite rate used for allowance for funds used during construction	5.73%		5.73%	5.73%		5.73%
AFUDC, net of income taxes			0.07%	0.00%	10.80%
Interest, net of amounts capitalized			$ 46	$ 53	$ 52
Net cash paid for capitalized interest			0	0	6
MISSISSIPPI POWER CO
Accounting Policies [Line Items]
Allowance for equity funds used during construction			$ 72	$ 124	$ 110
Composite rate used for allowance for funds used during construction	6.50%		6.70%	6.50%	5.99%
Interest, net of amounts capitalized			$ 65	$ 50	$ 45
Net cash paid for capitalized interest			29	49	$ 66
SOUTHERN Co GAS
Accounting Policies [Line Items]
Interest, net of amounts capitalized	$ 135		$ 223
Atlanta Gas Light
Accounting Policies [Line Items]
Allowance for funds under construction rate (as percent)	4.05%		8.10%
Chattanooga Gas
Accounting Policies [Line Items]
Allowance for funds under construction rate (as percent)	3.71%		7.41%
Elizabethtown Gas
Accounting Policies [Line Items]
Allowance for funds under construction rate (as percent)	0.84%		1.56%
Nicor Gas
Accounting Policies [Line Items]
Allowance for funds under construction rate (as percent)	1.50%		1.22%
Predecessor | SOUTHERN Co GAS
Accounting Policies [Line Items]
Interest, net of amounts capitalized		$ 119		$ 181
Predecessor | Atlanta Gas Light
Accounting Policies [Line Items]
Allowance for funds under construction rate (as percent)		4.05%			8.10%
Predecessor | Chattanooga Gas
Accounting Policies [Line Items]
Allowance for funds under construction rate (as percent)		3.71%			7.41%
Predecessor | Elizabethtown Gas
Accounting Policies [Line Items]
Allowance for funds under construction rate (as percent)		0.84%			1.69%
Predecessor | Nicor Gas
Accounting Policies [Line Items]
Allowance for funds under construction rate (as percent)		1.50%			0.82%