Consolidated Entities and Equity Method Investments - Balance Sheet Information (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	$ 1,580	$ 1,513
Southern Company Gas
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	1,538	1,477
SNG | Southern Company Gas
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	1,261	1,262
Current assets	104	82
Property, plant, and equipment	2,606	2,439
Deferred charges and other assets	121	121
Total Assets	2,831	2,642
Current liabilities	103	110
Long-term debt	1,103	1,102
Other deferred charges and other liabilities	212	76
Total Liabilities	1,418	1,288
Total Stockholders' Equity	1,413	1,354
Total Liabilities and Stockholders' Equity	2,831	2,642
PennEast Pipeline | Southern Company Gas
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	71	57
Atlantic Coast Pipeline | Southern Company Gas
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	83	41
Other | Southern Company Gas
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	$ 123	$ 117