Summary of Significant Accounting Policies - Transmission Receivables/Payables, Cash and Accumulated OCI (loss) (Details) $ in Millions		6 Months Ended	12 Months Ended
		Dec. 31, 2016 USD ($)	Dec. 31, 2017 USD ($) counterparty customer	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2014 USD ($)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Required collateral credit rating downgrade		$ 0	$ 8	$ 0
Stockholders' equity		26,612	$ 25,528	26,612	$ 21,982	$ 8,001	$ 20,926
Original maturities of temporary cash investments			90 days
Change in Accumulated OCI (loss) balances [Roll Forward]
Beginning Balance			$ (180)
Current period change			(9)	(50)	(2)
Ending Balance		(180)	(189)	(180)
Other comprehensive income (loss)			(9)	(50)	(2)
Qualifying Hedges
Change in Accumulated OCI (loss) balances [Roll Forward]
Beginning Balance			(115)
Current period change			(4)
Ending Balance		(115)	(119)	(115)
Pension and Other Postretirement Benefit Plans
Change in Accumulated OCI (loss) balances [Roll Forward]
Beginning Balance			(65)
Current period change			(5)
Ending Balance		(65)	(70)	(65)
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity		(180)	(189)	(180)	(130)		(128)
Change in Accumulated OCI (loss) balances [Roll Forward]
Current period change			(9)	(50)	(2)
ALABAMA POWER CO
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity		6,323	$ 6,829	6,323	5,992		5,752
Original maturities of temporary cash investments			90 days
Change in Accumulated OCI (loss) balances [Roll Forward]
Beginning Balance			$ (30)
Current period change			4	2	(3)
Ending Balance		(30)	(26)	(30)
Other comprehensive income (loss)			4	2	(3)
ALABAMA POWER CO | Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity		(30)	(26)	(30)	(32)		(29)
Change in Accumulated OCI (loss) balances [Roll Forward]
Current period change			4	2	(3)
GEORGIA POWER CO
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity		11,356	$ 11,931	11,356	10,719		10,421
Original maturities of temporary cash investments			90 days
Change in Accumulated OCI (loss) balances [Roll Forward]
Beginning Balance			$ (13)
Current period change			3	2	(7)
Ending Balance		(13)	(10)	(13)
Other comprehensive income (loss)			3	2	(7)
GEORGIA POWER CO | Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity		(13)	(10)	(13)	(15)		(8)
Change in Accumulated OCI (loss) balances [Roll Forward]
Current period change			3	2	(7)
GULF POWER CO
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity		1,389	$ 1,531	1,389	1,355		1,309
Original maturities of temporary cash investments			90 days
Change in Accumulated OCI (loss) balances [Roll Forward]
Beginning Balance			$ 1
Current period change			(1)	1	1
Ending Balance		1	0	1
Other comprehensive income (loss)			(1)	1	1
GULF POWER CO | Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity		1	0	1	0		(1)
Change in Accumulated OCI (loss) balances [Roll Forward]
Current period change			(1)	1	1
MISSISSIPPI POWER CO
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity		2,943	$ 1,358	2,943	2,359		2,084
Original maturities of temporary cash investments			90 days
Change in Accumulated OCI (loss) balances [Roll Forward]
Beginning Balance			$ (4)
Current period change				2	1
Ending Balance		(4)	(4)	(4)
Other comprehensive income (loss)			0	2	1
MISSISSIPPI POWER CO | Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity		(4)	(4)	(4)	(6)		(7)
Change in Accumulated OCI (loss) balances [Roll Forward]
Current period change				2	1
SOUTHERN POWER CO
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity		5,675	$ 6,498	5,675	3,264		1,971
Period of reimbursement of transmission costs			5 years
Restricted cash and cash equivalents noncurrent		13	$ 11	13
Original maturities of temporary cash investments			90 days
Change in Accumulated OCI (loss) balances [Roll Forward]
Beginning Balance			$ 35
Current period change			(10)	31	1
Other Comprehensive Income (Loss), Transfer from Service Company	[1]		27
Ending Balance		35	(2)	35
Other comprehensive income (loss)			(10)	31	1
SOUTHERN POWER CO | Qualifying Hedges
Change in Accumulated OCI (loss) balances [Roll Forward]
Beginning Balance			35
Current period change			(10)
Other Comprehensive Income (Loss), Transfer from Service Company			0
Ending Balance		35	25	35
SOUTHERN POWER CO | Pension and Other Postretirement Benefit Plans
Change in Accumulated OCI (loss) balances [Roll Forward]
Beginning Balance			0
Current period change			0
Other Comprehensive Income (Loss), Transfer from Service Company			27
Ending Balance		0	(27)	0
Other comprehensive income (loss)			27
Other Comprehensive Income (Loss), Transfer from Service Company, Net of Tax			9
SOUTHERN POWER CO | Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity		35	(2)	35	4		$ 3
Change in Accumulated OCI (loss) balances [Roll Forward]
Current period change			(10)	31	$ 1
Other Comprehensive Income (Loss), Transfer from Service Company	[1]		27
SOUTHERN Co GAS
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity		9,109	$ 9,022	9,109
Original maturities of temporary cash investments			90 days
Change in Accumulated OCI (loss) balances [Roll Forward]
Beginning Balance			$ 26
Current period change		26	(5)
Ending Balance		26	20	26
Other comprehensive income (loss)		26	(5)
SOUTHERN Co GAS | Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' equity		26	20	$ 26
Change in Accumulated OCI (loss) balances [Roll Forward]
Current period change		$ 26	$ (5)
Atlanta Gas Light
Accumulated Other Comprehensive Income (Loss) [Line Items]
Concentration risk, number of customers | customer			15
Wholesale Gas Services | Accounts Receivable | Credit Concentration Risk | SOUTHERN Co GAS
Accumulated Other Comprehensive Income (Loss) [Line Items]
Number of top counterparties | counterparty			20
Wholesale Services | Accounts Receivable | Credit Concentration Risk | SOUTHERN Co GAS
Accumulated Other Comprehensive Income (Loss) [Line Items]
Concentration risk (as percent)			48.00%
Accounts receivable			$ 203

[1] Amount includes carry-over OCI balance of $27 million in connection with the Company becoming a participant to the Southern Company qualified pension plan.