Income Taxes - Current and Deferred Income Tax Provisions (Details) - USD ($) $ in Millions	3 Months Ended								6 Months Ended		12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Federal —
Current											$ (62)	$ 1,184	$ (177)
Deferred											(6)	(342)	1,266
Total federal taxes											(68)	842	1,089
State —
Current											37	(108)	(33)
Deferred											173	217	138
Total state taxes											210	109	105
Income taxes											142	951	1,194
ALABAMA POWER CO
Federal —
Current											136	103	110
Deferred											336	339	320
Total federal taxes											472	442	430
State —
Current											23	20	8
Deferred											73	69	68
Total state taxes											96	89	76
Income taxes											568	531	506
GEORGIA POWER CO
Federal —
Current											256	391	515
Deferred											504	319	176
Total federal taxes											760	710	691
State —
Current											116	6	81
Deferred											(46)	64	(3)
Total state taxes											70	70	78
Income taxes											830	780	769
GULF POWER CO
Federal —
Current											19	34	(3)
Deferred											58	45	80
Total federal taxes											77	79	77
State —
Current											(1)	0	5
Deferred											14	12	10
Total state taxes											13	12	15
Income taxes											90	91	92
MISSISSIPPI POWER CO
Federal —
Current											194	(31)	(768)
Deferred											(753)	(60)	704
Total federal taxes											(559)	(91)	(64)
State —
Current											0	(6)	(81)
Deferred											27	(7)	73
Total state taxes											27	(13)	(8)
Income taxes											(532)	(104)	(72)
SOUTHERN POWER CO
Federal —
Current											(566)	928	12
Deferred											(312)	(1,098)	10
Total federal taxes											(878)	(170)	22
State —
Current											(110)	(60)	(32)
Deferred											49	35	31
Total state taxes											(61)	(25)	(1)
Income taxes	$ (810)	$ (39)	$ (38)	$ (52)	$ (29)	$ (102)	$ (41)	$ (23)			(939)	$ (195)	21
SOUTHERN Co GAS
Federal —
Current									$ 0		103
Deferred									65		170
Total federal taxes									65		273
State —
Current									(16)		27
Deferred									27		67
Total state taxes									11		94
Income taxes									$ 76		$ 367
Predecessor | SOUTHERN Co GAS
Federal —
Current										$ 67			(13)
Deferred										8			198
Total federal taxes										75			185
State —
Current										12			10
Deferred										0			18
Total state taxes										12			28
Income taxes										$ 87			$ 213