Financing - Schedule of Credit Arrangements (Details)	Sep. 30, 2018 USD ($)
Line of Credit Facility [Line Items]
Expires, 2018	$ 20,000,000
Expires, 2019	188,000,000
Expires, 2020	735,000,000
Expires, 2022	7,200,000,000
Total	8,143,000,000
Unused	8,101,000,000
Executable Term Loans, One Year	45,000,000
Expires Within One Year, Term Out	45,000,000
Expires Within One Year, No Term Out	63,000,000
Southern Company
Line of Credit Facility [Line Items]
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
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, 2018	0
Expires, 2019	33,000,000
Expires, 2020	500,000,000
Expires, 2022	800,000,000
Total	1,333,000,000
Unused	1,333,000,000
Executable Term Loans, One Year	0
Expires Within One Year, Term Out	0
Expires Within One Year, No Term Out	33,000,000
GEORGIA POWER CO
Line of Credit Facility [Line Items]
Expires, 2018	0
Expires, 2019	0
Expires, 2020	0
Expires, 2022	1,750,000,000
Total	1,750,000,000
Unused	1,736,000,000
Executable Term Loans, One Year	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, 2018	20,000,000
Expires, 2019	25,000,000
Expires, 2020	235,000,000
Expires, 2022	0
Total	280,000,000
Unused	280,000,000
Executable Term Loans, One Year	45,000,000
Expires Within One Year, Term Out	45,000,000
Expires Within One Year, No Term Out	0
MISSISSIPPI POWER CO
Line of Credit Facility [Line Items]
Expires, 2018	0
Expires, 2019	100,000,000
Expires, 2020	0
Expires, 2022	0
Total	100,000,000
Unused	100,000,000
Executable Term Loans, One Year	0
Expires Within One Year, Term Out	0
Expires Within One Year, No Term Out	0
SOUTHERN POWER CO
Line of Credit Facility [Line Items]
Expires, 2018	0
Expires, 2019	0
Expires, 2020	0
Expires, 2022	750,000,000
Total	750,000,000
Unused	728,000,000
Executable Term Loans, One Year	0
Expires Within One Year, Term Out	0
Expires Within One Year, No Term Out	0
SOUTHERN POWER CO | Standby Letters of Credit
Line of Credit Facility [Line Items]
Total	120,000,000
Unused	22,000,000
SOUTHERN Co GAS
Line of Credit Facility [Line Items]
Expires, 2018	0
Expires, 2019	0
Expires, 2020	0
Expires, 2022	1,900,000,000
Total	1,900,000,000
Unused	1,895,000,000
Executable Term Loans, One Year	0
Expires Within One Year, Term Out	0
Expires Within One Year, No Term Out	0
Other
Line of Credit Facility [Line Items]
Expires, 2018	0
Expires, 2019	30,000,000
Expires, 2020	0
Expires, 2022	0
Total	30,000,000
Unused	30,000,000
Executable Term Loans, One Year	0
Expires Within One Year, Term Out	0
Expires Within One Year, No Term Out	30,000,000
Southern Company Gas Capital
Line of Credit Facility [Line Items]
Total	1,400,000,000.0
Nicor Gas
Line of Credit Facility [Line Items]
Total	$ 500,000,000