Variable Interest Entity and Equity Method Investments - Income Statement Equity Method Investment Carrying Amount (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	$ 31	$ 28	$ 72	$ 67
SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	31	29	74	68
SOUTHERN Co GAS | SNG
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	27	24	66	58
SOUTHERN Co GAS | PennEast Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	1	1	2	4
SOUTHERN Co GAS | Atlantic Coast Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	1	2	3	3
SOUTHERN Co GAS | Triton
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	1	2	2	2
SOUTHERN Co GAS | Horizon Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	$ 1	$ 0	$ 1	$ 1