Summary of Significant Accounting Policies - Transmission Receivables/Payables, Cash and Accumulated OCI (loss) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) counterparty customer	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 26,612	$ 21,982	$ 20,926	$ 19,764
Original maturities of temporary cash investments	90 days
Change in Accumulated OCI (loss) balances [Roll Forward]
Beginning Balance	$ (130)
Current period change	(50)	(2)	(53)
Ending Balance	(180)	(130)
Other Comprehensive Income (Loss), Net of Tax	(50)	(2)	(53)
Qualifying Hedges
Change in Accumulated OCI (loss) balances [Roll Forward]
Beginning Balance	(48)
Current period change	(67)
Ending Balance	(115)	(48)
Marketable Securities
Change in Accumulated OCI (loss) balances [Roll Forward]
Beginning Balance	0
Current period change	0
Ending Balance	0	0
Pension and Other Postretirement Benefit Plans
Change in Accumulated OCI (loss) balances [Roll Forward]
Beginning Balance	(82)
Current period change	17
Ending Balance	(65)	(82)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(180)	(130)	(128)	(75)
Change in Accumulated OCI (loss) balances [Roll Forward]
Current period change	$ (50)	(2)	(53)
Southern Company Gas [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Original maturities of temporary cash investments	90 days
Atlanta Gas Light
Accumulated Other Comprehensive Income (Loss) [Line Items]
Concentration Risk, Number of Customers | customer	14
Southern Power [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 5,675	3,264	1,971	1,564
Period of reimbursement of transmission costs	5 years
Restricted cash and cash equivalents noncurrent	$ 13	5
Original maturities of temporary cash investments	90 days
Change in Accumulated OCI (loss) balances [Roll Forward]
Beginning Balance	$ 4
Current period change	31	1
Ending Balance	35	4
Other Comprehensive Income (Loss), Net of Tax	31	1	0
Southern Power [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	35	4	3	3
Change in Accumulated OCI (loss) balances [Roll Forward]
Current period change	31	1
Mississippi Power [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 2,943	2,359	2,084	2,177
Original maturities of temporary cash investments	90 days
Change in Accumulated OCI (loss) balances [Roll Forward]
Beginning Balance	$ (6)
Current period change	2	1	1
Ending Balance	(4)	(6)
Other Comprehensive Income (Loss), Net of Tax	2	1	1
Mississippi Power [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(4)	(6)	(7)	(8)
Change in Accumulated OCI (loss) balances [Roll Forward]
Current period change	2	1	1
Gulf Power [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 1,389	1,355	1,309	1,235
Original maturities of temporary cash investments	90 days
Change in Accumulated OCI (loss) balances [Roll Forward]
Beginning Balance	$ 0
Current period change	1	1
Ending Balance	1	0
Other Comprehensive Income (Loss), Net of Tax	1	1	0
Gulf Power [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	1	0	(1)	(1)
Change in Accumulated OCI (loss) balances [Roll Forward]
Current period change	1	1
Alabama Power [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 6,323	5,992	5,752	5,502
Original maturities of temporary cash investments	90 days
Change in Accumulated OCI (loss) balances [Roll Forward]
Beginning Balance	$ (32)
Current period change	2	(3)	(3)
Ending Balance	(30)	(32)
Other Comprehensive Income (Loss), Net of Tax	2	(3)	(3)
Alabama Power [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(30)	(32)	(29)	(26)
Change in Accumulated OCI (loss) balances [Roll Forward]
Current period change	2	(3)	(3)
Georgia Power [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	$ 11,356	10,719	10,421	9,591
Original maturities of temporary cash investments	90 days
Change in Accumulated OCI (loss) balances [Roll Forward]
Beginning Balance	$ (15)
Current period change	2	(7)	(3)
Ending Balance	(13)	(15)
Other Comprehensive Income (Loss), Net of Tax	2	(7)	(3)
Georgia Power [Member] | Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest	(13)	(15)	(8)	$ (5)
Change in Accumulated OCI (loss) balances [Roll Forward]
Current period change	$ 2	$ (7)	$ (3)
Wholesale Gas Services [Member] | Accounts Receivable [Member] | Credit Concentration Risk [Member] | Southern Company Gas [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Number of top counterparties | counterparty	20
Wholesale Services [Member] | Accounts Receivable [Member] | Credit Concentration Risk [Member] | Southern Company Gas [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Concentration risk (as percent)	46.00%
Accounts receivable	$ 205