Joint Ownership Agreements - Equity Method Investments - Balance Sheet Information (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	$ 1,513	$ 1,549
SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	1,477	1,541
SNG | SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Current assets	82	95
Equity investments in unconsolidated subsidiaries	1,262	1,394
Property, plant, and equipment	2,439	2,451
Noncurrent assets	121	129
Total Assets	2,642	2,675
Current liabilities	110	588
Long-term debt	1,102	706
Noncurrent liabilities	76	22
Total Liabilities	1,288	1,316
Total Stockholders' Equity	1,354	1,359
Total Liabilities and Stockholders' Equity	2,642	2,675
Tax Cuts and Jobs Act of 2017, change in enacted tax rate, decrease in equity method investments	104
Triton | SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	42	44
Horizon Pipeline | SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	30	30
PennEast Pipeline | SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	57	22
Atlantic Coast Pipeline | SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	41	33
Pivotal JAX LNG, LLC | SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	44	16
Other | SOUTHERN Co GAS
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	$ 1	$ 2