Joint Ownership Agreements - Income Statement Equity Method Investment Carrying Amount (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Schedule of Equity Method Investments [Line Items]
Earnings (loss) from equity method investments	$ 28	$ (1)	$ 67	$ (1)
SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Earnings (loss) from equity method investments	29		68
SOUTHERN Co GAS | Predecessor
Schedule of Equity Method Investments [Line Items]
Earnings (loss) from equity method investments		1		2
SOUTHERN Co GAS | SNG
Schedule of Equity Method Investments [Line Items]
Earnings (loss) from equity method investments	24		58
SOUTHERN Co GAS | SNG | Predecessor
Schedule of Equity Method Investments [Line Items]
Earnings (loss) from equity method investments		0		0
SOUTHERN Co GAS | Triton
Schedule of Equity Method Investments [Line Items]
Earnings (loss) from equity method investments	2		2
SOUTHERN Co GAS | Triton | Predecessor
Schedule of Equity Method Investments [Line Items]
Earnings (loss) from equity method investments		1		1
SOUTHERN Co GAS | Atlantic Coast Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings (loss) from equity method investments	0		1
SOUTHERN Co GAS | Atlantic Coast Pipeline | Predecessor
Schedule of Equity Method Investments [Line Items]
Earnings (loss) from equity method investments		0		1
SOUTHERN Co GAS | PennEast Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings (loss) from equity method investments	1		4
SOUTHERN Co GAS | PennEast Pipeline | Predecessor
Schedule of Equity Method Investments [Line Items]
Earnings (loss) from equity method investments		0		0
SOUTHERN Co GAS | Triton
Schedule of Equity Method Investments [Line Items]
Earnings (loss) from equity method investments	$ 2		$ 3
SOUTHERN Co GAS | Triton | Predecessor
Schedule of Equity Method Investments [Line Items]
Earnings (loss) from equity method investments		$ 0		$ 0