Summary of Significant Accounting Policies - Affiliate Transactions (Narrative) (Details)	12 Months Ended
	Dec. 31, 2018 USD ($) base	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Oct. 31, 2016 location
Georgia Power
Related Party Transaction [Line Items]
Number of solar power generation facilities	2			2
Southern Nuclear Operating Company, Inc. | Alabama Power
Related Party Transaction [Line Items]
Costs for services	$ 247,000,000	$ 248,000,000	$ 249,000,000
Southern Nuclear Operating Company, Inc. | Georgia Power
Related Party Transaction [Line Items]
Costs for services	780,000,000	675,000,000	666,000,000
Southern Natural Gas Company, LLC | Alabama Power
Related Party Transaction [Line Items]
Costs for services	8,000,000	9,000,000	2,000,000
Southern Natural Gas Company, LLC | Georgia Power
Related Party Transaction [Line Items]
Costs for services	101,000,000	102,000,000	35,000,000
Due from affiliate	142,000,000
Southern Natural Gas Company, LLC | Southern Company Gas
Related Party Transaction [Line Items]
Costs for services	32,000,000	32,000,000	15,000,000
Due from affiliate	122,000,000
Southern Natural Gas Company, LLC | Southern Power
Related Party Transaction [Line Items]
Costs for services	25,000,000	25,000,000	7,000,000
Gulf Power | Alabama Power
Related Party Transaction [Line Items]
Amounts received related to tariff	11,000,000	11,000,000	12,000,000
Gulf Power | Georgia Power
Related Party Transaction [Line Items]
Costs for services	8,000,000	11,000,000	8,000,000
Gulf Power | Mississippi Power
Related Party Transaction [Line Items]
Costs for services	31,000,000	31,000,000	26,000,000
PowerSecure International, Inc. | Alabama Power
Related Party Transaction [Line Items]
Costs for services	24,000,000	11,000,000	0
PowerSecure International, Inc. | Georgia Power
Related Party Transaction [Line Items]
Costs for services	32,000,000
Southern Power | Georgia Power
Related Party Transaction [Line Items]
Costs for services	216,000,000	235,000,000	265,000,000
Southern Company Services, Inc. | Alabama Power
Related Party Transaction [Line Items]
Costs for services	508,000,000	479,000,000	460,000,000
Southern Company Services, Inc. | Georgia Power
Related Party Transaction [Line Items]
Costs for services	653,000,000	625,000,000	606,000,000
Southern Company Services, Inc. | Southern Company Gas
Related Party Transaction [Line Items]
Costs for services	194,000,000	63,000,000	17,000,000
Southern Company Services, Inc. | Mississippi Power
Related Party Transaction [Line Items]
Costs for services	104,000,000	140,000,000	231,000,000
Southern Company Services, Inc. | Southern Power
Related Party Transaction [Line Items]
Costs for services	98,000,000	218,000,000	193,000,000
Fuel Purchases | Mississippi Power | Alabama Power
Related Party Transaction [Line Items]
Costs for services	0	0	0
Non-Fuel Expense | Mississippi Power | Alabama Power
Related Party Transaction [Line Items]
Costs for services	8,000,000	9,000,000	13,000,000
Purchased Power from Affiliates | Southern Power
Related Party Transaction [Line Items]
Costs for services	215,000,000	233,000,000	258,000,000
Operating Lease PPA | Southern Power
Related Party Transaction [Line Items]
Costs for services	65,000,000	81,000,000	109,000,000
Electric Transmission | Southern Company Services, Inc. | Southern Power
Related Party Transaction [Line Items]
Costs for services	$ 12,000,000	$ 13,000,000	$ 11,000,000
Plant Scherer (coal) Unit 3 | Gulf Power | Georgia Power
Related Party Transaction [Line Items]
Proportionate share of related non-fuel expenses (percent)	25.00%