Joint Ownership Agreements - Equity Method Investments - Income Statement Information (Details) - USD ($) $ in Millions	4 Months Ended	6 Months Ended		9 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2016	Jun. 30, 2016	Oct. 03, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments				$ 15	$ 59	$ 0	$ 0
Southern Company Gas [Member] | Successor [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		$ 60
Southern Company Gas [Member] | Successor [Member] | SNG [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		56
Revenues	$ 230
Operating income	138
Net income	$ 115
Southern Company Gas [Member] | Successor [Member] | Triton [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		2
Southern Company Gas [Member] | Successor [Member] | Horizon Pipeline [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		1
Southern Company Gas [Member] | Successor [Member] | Atlantic Coast Pipeline [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		$ 1
Southern Company Gas [Member] | Predecessor [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			$ 2			6	8
Southern Company Gas [Member] | Predecessor [Member] | SNG [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			0			0	0
Southern Company Gas [Member] | Predecessor [Member] | Triton [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			1			4	6
Southern Company Gas [Member] | Predecessor [Member] | Horizon Pipeline [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			1			2	2
Southern Company Gas [Member] | Predecessor [Member] | Atlantic Coast Pipeline [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			$ 0			$ 0	$ 0