Joint Ownership Agreements - Balance Sheet Equity Method Investment Carrying Amount (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	$ 1,632	$ 1,549
SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	1,610	1,541
SNG | SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	1,405	1,394
Triton | SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	43	44
Atlantic Coast Pipeline | SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	31	30
PennEast Pipeline | SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	45	22
Triton | SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	53	33
Pivotal JAX LNG, LLC | SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	32	16
Other | SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	$ 1	$ 2