Summary of Significant Accounting Policies - Intangible Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Line Items]
Other liabilities noncurrent	$ 691	$ 880
SOUTHERN Co GAS
Accounting Policies [Line Items]
Intangible liabilities, accumulated amortization	50
Other liabilities noncurrent	88	$ 127
Finite-Lived Intangible Liabilities, Amortization Expense, Maturity Schedule [Abstract]
2018	24
2019	17
Intangible Liabilities | SOUTHERN Co GAS
Accounting Policies [Line Items]
Other liabilities noncurrent	$ 91