Financing - Schedule of Credit Arrangements (Details) - USD ($)	Sep. 30, 2017	May 31, 2017
Line of Credit Facility [Line Items]
Expires, 2017	$ 130,000,000
Expires, 2018	260,000,000
Expires, 2019	25,000,000
Expires, 2020	530,000,000
Expires, 2022	7,200,000,000
Total	8,145,000,000
Unused	8,066,000,000
Executable Term Loans, One Year	65,000,000
Executable Term Loans, Two Years	0
Expires Within One Year, Term Out	20,000,000
Expires Within One Year, No Term Out	185,000,000
Southern Company
Line of Credit Facility [Line Items]
Expires, 2017	0
Expires, 2018	0
Expires, 2019	0
Expires, 2020	0
Expires, 2022	2,000,000,000
Total	2,000,000,000	$ 1,250,000,000.00
Unused	2,000,000,000
Executable Term Loans, One Year	0
Executable Term Loans, Two Years	0
Expires Within One Year, Term Out	0
Expires Within One Year, No Term Out	0
ALABAMA POWER CO
Line of Credit Facility [Line Items]
Expires, 2017	0
Expires, 2018	35,000,000
Expires, 2019	0
Expires, 2020	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
Expires Within One Year, Term Out	0
Expires Within One Year, No Term Out	35,000,000
GEORGIA POWER CO
Line of Credit Facility [Line Items]
Expires, 2017	0
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
Expires Within One Year, Term Out	0
Expires Within One Year, No Term Out	0
GULF POWER CO
Line of Credit Facility [Line Items]
Expires, 2017	30,000,000
Expires, 2018	195,000,000
Expires, 2019	25,000,000
Expires, 2020	30,000,000
Expires, 2022	0
Total	280,000,000
Unused	280,000,000
Executable Term Loans, One Year	45,000,000
Executable Term Loans, Two Years	0
Expires Within One Year, Term Out	0
Expires Within One Year, No Term Out	40,000,000
MISSISSIPPI POWER CO
Line of Credit Facility [Line Items]
Expires, 2017	100,000,000
Expires, 2018	0
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
Expires Within One Year, Term Out	0
Expires Within One Year, No Term Out	100,000,000
SOUTHERN POWER CO
Line of Credit Facility [Line Items]
Expires, 2017	0
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
Executable Term Loans, One Year	0
Executable Term Loans, Two Years	0
Expires Within One Year, Term Out	0
Expires Within One Year, No Term Out	0
SOUTHERN Co GAS
Line of Credit Facility [Line Items]
Expires, 2017	0
Expires, 2018	0
Expires, 2019	0
Expires, 2020	0
Expires, 2022	1,900,000,000
Total	1,900,000,000
Unused	1,861,000,000
Executable Term Loans, One Year	0
Executable Term Loans, Two Years	0
Expires Within One Year, Term Out	0
Expires Within One Year, No Term Out	0
Other
Line of Credit Facility [Line Items]
Expires, 2017	0
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
Expires Within One Year, Term Out	20,000,000
Expires Within One Year, No Term Out	10,000,000
Southern Company Gas Capital
Line of Credit Facility [Line Items]
Total	1,200,000,000.0	1,200,000,000.0
Nicor Gas
Line of Credit Facility [Line Items]
Total	700,000,000	$ 700,000,000
Standby Letters of Credit | SOUTHERN POWER CO
Line of Credit Facility [Line Items]
Total	120,000,000
Unused	9,000,000
Long-term line of credit	$ 111,000,000