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, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Line Items]
Allowance for equity funds used during construction			$ 202	$ 226	$ 245
AFUDC, net of income taxes			11.40%	12.80%	16.00%
Interest, net of amounts capitalized			$ 1,100	$ 809	$ 732
Net cash paid for capitalized interest			125	124	111
Georgia Power [Member]
Accounting Policies [Line Items]
Allowance for equity funds used during construction			$ 48	$ 40	$ 45
Composite rate used for allowance for funds used during construction	6.90%		6.90%	6.50%	5.60%
Allowance for Funds Used During Construction, Capitalized Interest			$ 68	$ 56	$ 62
AFUDC, net of income taxes			4.60%	3.90%	4.60%
Interest, net of amounts capitalized			$ 375	$ 353	$ 319
Net cash paid for capitalized interest			20	16	18
Alabama Power [Member]
Accounting Policies [Line Items]
Allowance for equity funds used during construction			$ 28	$ 60	$ 49
Composite rate used for allowance for funds used during construction	8.40%		8.40%	8.70%	8.80%
AFUDC, net of income taxes			4.20%	9.30%	7.90%
Interest, net of amounts capitalized			$ 277	$ 250	$ 231
Net cash paid for capitalized interest			11	22	18
Mississippi Power [Member]
Accounting Policies [Line Items]
Allowance for equity funds used during construction			$ 124	$ 110	$ 136
Composite rate used for allowance for funds used during construction	6.50%		6.50%	5.99%	6.91%
Interest, net of amounts capitalized			$ 50	$ 45	$ 7
Net cash paid for capitalized interest			$ 49	$ 66	$ 69
Gulf Power [Member]
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			0.00%	10.80%	10.93%
Interest, net of amounts capitalized			$ 53	$ 52	$ 48
Net cash paid for capitalized interest			$ 0	6	5
Successor [Member] | Southern Company Gas [Member]
Accounting Policies [Line Items]
Interest, net of amounts capitalized	$ 135
Successor [Member] | Atlanta Gas Light
Accounting Policies [Line Items]
Allowance for funds under construction rate (as percent)	4.05%
Successor [Member] | Chattanooga Gas [Member]
Accounting Policies [Line Items]
Allowance for funds under construction rate (as percent)	3.71%
Successor [Member] | Elizabethtown Gas [Member]
Accounting Policies [Line Items]
Allowance for funds under construction rate (as percent)	0.84%
Successor [Member] | Nicor Gas [Member]
Accounting Policies [Line Items]
Allowance for funds under construction rate (as percent)	1.50%
Predecessor [Member] | Southern Company Gas [Member]
Accounting Policies [Line Items]
Interest, net of amounts capitalized		$ 119		$ 181	$ 187
Predecessor [Member] | Atlanta Gas Light
Accounting Policies [Line Items]
Allowance for funds under construction rate (as percent)		4.05%		8.10%	8.10%
Predecessor [Member] | Chattanooga Gas [Member]
Accounting Policies [Line Items]
Allowance for funds under construction rate (as percent)		3.71%		7.41%	7.41%
Predecessor [Member] | Elizabethtown Gas [Member]
Accounting Policies [Line Items]
Allowance for funds under construction rate (as percent)		0.84%		1.69%	0.44%
Predecessor [Member] | Nicor Gas [Member]
Accounting Policies [Line Items]
Allowance for funds under construction rate (as percent)		1.50%		0.82%	0.24%