Consolidated Entities and 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, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments				$ 15	$ 148	$ 106	$ 59
Southern Company Gas
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		$ 60			148	106
Southern Company Gas | SNG
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		56			131	88
Revenues	$ 230				604	544
Operating income	137				310	242
Net income	$ 115				261	175
Southern Company Gas | PennEast Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		0			5	6
Southern Company Gas | Atlantic Coast Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		1			7	6
Southern Company Gas | Other
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		$ 3			$ 5	$ 6
Southern Company Gas | Predecessor
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			$ 2
Southern Company Gas | Predecessor | SNG
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			0
Southern Company Gas | Predecessor | PennEast Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			0
Southern Company Gas | Predecessor | Atlantic Coast Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			0
Southern Company Gas | Predecessor | Other
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			$ 2