Joint Ownership Agreements - Income Statement Equity Method Investment Carrying Amount (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Jun. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	$ 32	$ 29		$ 100	$ 28
SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	32	29		100
SOUTHERN Co GAS | Predecessor
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			$ 2
SOUTHERN Co GAS | SNG
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	28	27		86
SOUTHERN Co GAS | SNG | Predecessor
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			0
SOUTHERN Co GAS | Triton
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		0		5
SOUTHERN Co GAS | Triton | Predecessor
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			0
SOUTHERN Co GAS | PennEast Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	1	1		4
SOUTHERN Co GAS | PennEast Pipeline | Predecessor
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			1
SOUTHERN Co GAS | Atlantic Coast Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	1	1		3
SOUTHERN Co GAS | Atlantic Coast Pipeline | Predecessor
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			1
SOUTHERN Co GAS | Horizon Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	$ 1	$ 0		$ 2
SOUTHERN Co GAS | Horizon Pipeline | Predecessor
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			$ 0