Consolidated Entities and Equity Method Investments - GAS Equity Method Investments Narrative (Details) $ in Millions	6 Months Ended		10 Months Ended	12 Months Ended
	Dec. 31, 2016 USD ($)	Jun. 30, 2016 USD ($)	Oct. 31, 2016 USD ($)	Dec. 31, 2018 USD ($) gal	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015	Dec. 31, 2014 mi Bcf
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments			$ 15	$ 148	$ 106	$ 59
Distributions to noncontrolling interests				153	119	$ 72
Southern Company Gas
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	$ 60			148	106
Distributions to noncontrolling interests	15			$ 0	0
Southern Company Gas | Southern Natural Gas Company, LLC
Schedule of Equity Method Investments [Line Items]
Ownership percentage, equity method investment				50.00%
Southern Company Gas | PennEast Pipeline
Schedule of Equity Method Investments [Line Items]
Ownership percentage, equity method investment								20.00%
Pipeline infrastructure | mi								118
Southern Company Gas | Atlantic Coast Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments	1			$ 7	$ 6
Ownership percentage, equity method investment								5.00%
Pipeline infrastructure | mi								594
Southern Company Gas | Pivotal JAX LNG, LLC
Schedule of Equity Method Investments [Line Items]
Ownership percentage, equity method investment				50.00%
Southstar | Southern Company Gas
Schedule of Equity Method Investments [Line Items]
Agreement to purchase remaining interest			$ 160
Piedmont | Southstar
Schedule of Equity Method Investments [Line Items]
Payments of ordinary dividends to noncontrolling interests	$ 15
Predecessor | Southern Company Gas
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		$ 2
Distributions to noncontrolling interests		19
Predecessor | Southern Company Gas | Atlantic Coast Pipeline
Schedule of Equity Method Investments [Line Items]
Earnings from equity method investments		0
Predecessor | Piedmont | Southstar
Schedule of Equity Method Investments [Line Items]
Distributions to noncontrolling interests		$ 19
Minimum | Southern Company Gas | PennEast Pipeline
Schedule of Equity Method Investments [Line Items]
Natural gas pipeline capacity (volume) | Bcf								1.0
Minimum | Southern Company Gas | Atlantic Coast Pipeline
Schedule of Equity Method Investments [Line Items]
Natural gas pipeline capacity (volume) | Bcf								1.5
Southstar | Southstar | Southern Company Gas
Schedule of Equity Method Investments [Line Items]
Ownership percentage of noncontrolling interest	85.00%					85.00%
Southstar | Southstar | Piedmont
Schedule of Equity Method Investments [Line Items]
Ownership percentage of noncontrolling interest							15.00%
Georgia Natural Gas | Georgia Natural Gas | Southern Company Gas
Schedule of Equity Method Investments [Line Items]
Noncontrolling ownership percentage held by parent							100.00%
Liquefied Natural Gas (LNG) | Southern Company Gas | Pivotal JAX LNG, LLC
Schedule of Equity Method Investments [Line Items]
Storage facility capacity | gal				2,000,000
Liquefied Natural Gas (LNG) | Minimum | Southern Company Gas | Pivotal JAX LNG, LLC
Schedule of Equity Method Investments [Line Items]
Storage facility production capacity | gal				120,000