Joint Ownership Agreements - Balance Sheet Equity Method Investment Carrying Amount (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	$ 1,615	$ 1,549
Southern Company Gas [Member]
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	1,604	1,541
SNG [Member] | Southern Company Gas [Member]
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	1,430	1,394
Triton [Member] | Southern Company Gas [Member]
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	44	44
Horizon Pipeline [Member] | Southern Company Gas [Member]
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	31	30
PennEast Pipeline [Member] | Southern Company Gas [Member]
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	30	22
Atlantic Coast Pipeline [Member] | Southern Company Gas [Member]
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	42	33
Pivotal JAX LNG, LLC [Member] | Southern Company Gas [Member]
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	26	16
Other [Member] | Southern Company Gas [Member]
Schedule of Equity Method Investments [Line Items]
Equity investments in unconsolidated subsidiaries	$ 1	$ 2