Financing - Schedule of Credit Arrangements (Details) - USD ($)	Jun. 30, 2017	May 31, 2017
Line of Credit Facility [Line Items]
Expires, 2017	$ 156,000,000
Expires, 2018	757,000,000
Expires, 2019	25,000,000
Expires, 2020	30,000,000
Expires, 2022	7,200,000,000
Total	8,168,000,000
Unused	8,011,000,000
Executable Term Loans, One Year	65,000,000
Executable Term Loans, Two Years	13,000,000
Expires Within One Year, Term Out	33,000,000
Expires Within One Year, No Term Out	195,000,000
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	3,000,000
Expires, 2018	532,000,000
Expires, 2019	0
Expires, 2020	0
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	113,000,000
Expires, 2018	0
Expires, 2019	0
Expires, 2020	0
Expires, 2022	0
Total	113,000,000
Unused	100,000,000
Executable Term Loans, One Year	0
Executable Term Loans, Two Years	13,000,000
Expires Within One Year, Term Out	13,000,000
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	675,000,000
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	1,849,000,000
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 [Member]
Line of Credit Facility [Line Items]
Expires, 2017	10,000,000
Expires, 2018	30,000,000
Expires, 2019	0
Expires, 2020	0
Expires, 2022	0
Total	40,000,000
Unused	40,000,000
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	20,000,000
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