Financing - Committed Credit Arrangements With Banks (Details) - USD ($)	Dec. 31, 2017	Nov. 30, 2017	Sep. 30, 2017	May 31, 2017	Apr. 30, 2017	Dec. 31, 2016
Credit arrangements by company
Expires, 2018	$ 195,000,000
Expires, 2019	25,000,000
Expires, 2020	725,000,000
Expires, 2022	7,200,000,000
Total	8,145,000,000
Unused	8,094,000,000
Executable term-loans, one year	65,000,000
Executable term-loans, two years	0
Due within one year, term out	40,000,000
Due within one year, no term out	155,000,000
Parent Company
Credit arrangements by company
Expires, 2018	0
Expires, 2019	0
Expires, 2020	0
Expires, 2022	2,000,000,000
Total	2,000,000,000
Unused	1,999,000,000
Executable term-loans, one year	0
Executable term-loans, two years	0
Due within one year, term out	0
Due within one year, no term out	0
ALABAMA POWER CO
Credit arrangements by company
Expires, 2018	35,000,000
Expires, 2019	0
Expires, 2020	500,000,000		$ 500,000,000
Expires, 2022	800,000,000
Total	1,335,000,000
Unused	1,335,000,000
Executable term-loans, one year	0
Executable term-loans, two years	0
Due within one year, term out	0
Due within one year, no term out	35,000,000
GEORGIA POWER CO
Credit arrangements by company
Expires, 2018	0
Expires, 2019	0
Expires, 2020	0
Expires, 2022	1,750,000,000
Total	1,750,000,000
Unused	1,732,000,000
Executable term-loans, one year	0
Executable term-loans, two years	0
Due within one year, term out	0
Due within one year, no term out	0
GULF POWER CO
Credit arrangements by company
Expires, 2018	30,000,000
Expires, 2019	25,000,000
Expires, 2020	225,000,000
Expires, 2022	0
Total	280,000,000	$ 195,000,000
Unused	280,000,000
Executable term-loans, one year	45,000,000
Executable term-loans, two years	0
Due within one year, term out	20,000,000
Due within one year, no term out	10,000,000
SOUTHERN Co GAS
Credit arrangements by company
Expires, 2018	0
Expires, 2019	0
Expires, 2020	0
Expires, 2022	1,900,000,000
Total	1,900,000,000
Unused	1,890,000,000
Executable term-loans, one year	0
Executable term-loans, two years	0
Due within one year, term out	0
Due within one year, no term out	0
MISSISSIPPI POWER CO
Credit arrangements by company
Expires, 2018	100,000,000
Expires, 2019	0
Expires, 2020	0
Expires, 2022	0
Total	100,000,000
Unused	100,000,000
Executable term-loans, one year	0
Executable term-loans, two years	0
Due within one year, term out	0
Due within one year, no term out	100,000,000
SOUTHERN POWER CO
Credit arrangements by company
Expires, 2018	0
Expires, 2019	0
Expires, 2020	0
Expires, 2022	750,000,000
Total	750,000,000				$ 600,000,000
Unused	728,000,000					$ 522,000,000
Executable term-loans, one year	0
Executable term-loans, two years	0
Due within one year, term out	0
Due within one year, no term out	0
Southern Company Gas Capital
Credit arrangements by company
Expires, 2018				$ 1,400,000,000
Expires, 2022	1,400,000,000
Unused	1,390,000,000
Nicor Gas
Credit arrangements by company
Expires, 2018				$ 500,000,000
Expires, 2022	500,000,000
Unused	500,000,000
Other Subsidiaries
Credit arrangements by company
Expires, 2018	30,000,000
Expires, 2019	0
Expires, 2020	0
Expires, 2022	0
Total	30,000,000
Unused	30,000,000
Executable term-loans, one year	20,000,000
Executable term-loans, two years	0
Due within one year, term out	20,000,000
Due within one year, no term out	10,000,000
Continuing Letter of Credit Facility | SOUTHERN POWER CO
Credit arrangements by company
Total	120,000,000
Unused	$ 82,000,000