Financing - Long-Term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Debt Disclosure [Line Items]
Senior notes	$ 32,725	$ 35,148
Long-term pollution control bonds	2,585	3,297
Long-term bank term loans	145	1,465
Junior Subordinated Debt
Debt Disclosure [Line Items]
Long-term debt	3,570	3,570
Alabama Power
Debt Disclosure [Line Items]
Senior notes	6,875	6,375
Long-term pollution control bonds	1,060	1,060
Long-term bank term loans	45	45
Georgia Power
Debt Disclosure [Line Items]
Senior notes	5,600	7,100
Long-term pollution control bonds	1,460	1,821
Long-term bank term loans	0	100
Georgia Power | Junior Subordinated Debt
Debt Disclosure [Line Items]
Long-term debt	270	270
Mississippi Power
Debt Disclosure [Line Items]
Senior notes	1,200	755
Long-term pollution control bonds	40	83
Long-term bank term loans	0	900
Southern Power
Debt Disclosure [Line Items]
Senior notes	5,050	5,459
Long-term bank term loans	0	420
Southern Company Gas
Debt Disclosure [Line Items]
Senior notes	4,000	4,157
Southern Company
Debt Disclosure [Line Items]
Senior notes	10,000	10,200
Southern Company | Junior Subordinated Debt
Debt Disclosure [Line Items]
Long-term debt	$ 3,300	$ 3,300