Joint Ownership Agreements - Income Statement Equity Method Investment Carrying Amount (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	$ 39	$ 0
Southern Company Gas [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	39
Southern Company Gas [Member] | Predecessor [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		1
Southern Company Gas [Member] | SNG [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	34
Southern Company Gas [Member] | SNG [Member] | Predecessor [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		0
Southern Company Gas [Member] | Horizon Pipeline [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	1
Southern Company Gas [Member] | Horizon Pipeline [Member] | Predecessor [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		1
Southern Company Gas [Member] | PennEast Pipeline [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	3
Southern Company Gas [Member] | PennEast Pipeline [Member] | Predecessor [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		0
Southern Company Gas [Member] | Atlantic Coast Pipeline [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	$ 1
Southern Company Gas [Member] | Atlantic Coast Pipeline [Member] | Predecessor [Member]
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		$ 0