Joint Ownership Agreements - Equity Method Investments - Income Statement Information (Details) - USD ($) $ in Millions	4 Months Ended	6 Months Ended		10 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2016	Jun. 30, 2016	Oct. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments				$ 15	$ 106	$ 59	$ 0
SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		$ 60			106
SOUTHERN Co GAS | SNG
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		56			88
Revenues	$ 230				544
Operating income	138				246
Net income	$ 115				175
SOUTHERN Co GAS | Triton
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		2			4
SOUTHERN Co GAS | Horizon Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		1			2
SOUTHERN Co GAS | Atlantic Coast Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		1			6
SOUTHERN Co GAS | Penn East Pipeline1
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		$ 0			$ 6
SOUTHERN Co GAS | Predecessor
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			$ 2				6
SOUTHERN Co GAS | Predecessor | SNG
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			0				0
SOUTHERN Co GAS | Predecessor | Triton
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			1				4
SOUTHERN Co GAS | Predecessor | Horizon Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			1				2
SOUTHERN Co GAS | Predecessor | Atlantic Coast Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			0				0
SOUTHERN Co GAS | Predecessor | Penn East Pipeline1
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			$ 0				$ 0