Financing - Schedule of Credit Arrangements (Details) - USD ($)	Mar. 31, 2016	Sep. 30, 2015
Line of Credit Facility [Line Items]
Expires, 2016	$ 390,000,000
Expires, 2017	40,000,000
Expires, 2018	1,665,000,000
Expires, 2020	4,400,000,000
Total	6,495,000,000
Unused	6,412,000,000
Executable Term Loans, One Year	95,000,000
Executable Term Loans, Two Years	15,000,000
Due Within One Year, Term Out	110,000,000
Due Within One Year, No Term Out	290,000,000
Bridge Agreement [Member]
Line of Credit Facility [Line Items]
Total		$ 8,100,000,000
Southern Company [Member]
Line of Credit Facility [Line Items]
Expires, 2016	0
Expires, 2017	0
Expires, 2018	1,000,000,000
Expires, 2020	1,250,000,000
Total	2,250,000,000
Unused	2,250,000,000
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 [Member]
Line of Credit Facility [Line Items]
Expires, 2016	40,000,000
Expires, 2017	0
Expires, 2018	500,000,000
Expires, 2020	800,000,000
Total	1,340,000,000
Unused	1,340,000,000
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	40,000,000
Georgia Power [Member]
Line of Credit Facility [Line Items]
Expires, 2016	0
Expires, 2017	0
Expires, 2018	0
Expires, 2020	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 [Member]
Line of Credit Facility [Line Items]
Expires, 2016	75,000,000
Expires, 2017	40,000,000
Expires, 2018	165,000,000
Expires, 2020	0
Total	280,000,000
Unused	280,000,000
Executable Term Loans, One Year	45,000,000
Executable Term Loans, Two Years	0
Due Within One Year, Term Out	45,000,000
Due Within One Year, No Term Out	40,000,000
Mississippi Power [Member]
Line of Credit Facility [Line Items]
Expires, 2016	205,000,000
Expires, 2017	0
Expires, 2018	0
Expires, 2020	0
Total	205,000,000
Unused	180,000,000
Executable Term Loans, One Year	30,000,000
Executable Term Loans, Two Years	15,000,000
Due Within One Year, Term Out	45,000,000
Due Within One Year, No Term Out	160,000,000
Southern Power [Member]
Line of Credit Facility [Line Items]
Expires, 2016	0
Expires, 2017	0
Expires, 2018	0
Expires, 2020	600,000,000
Total	600,000,000
Unused	560,000,000
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
Other [Member]
Line of Credit Facility [Line Items]
Expires, 2016	70,000,000
Expires, 2017	0
Expires, 2018	0
Expires, 2020	0
Total	70,000,000
Unused	70,000,000
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	$ 50,000,000