Summary of Significant Accounting Policies - Intangible Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Line Items]
Other liabilities noncurrent	$ 880	$ 678
Southern Company Gas [Member]
Accounting Policies [Line Items]
Intangible liabilities, accumulated amortization	21
Finite-Lived Intangible Liabilities, Amortization Expense, Maturity Schedule [Abstract]
2017	29
2018	24
2019	17
Intangible Liabilities | Southern Company Gas [Member]
Accounting Policies [Line Items]
Other liabilities noncurrent	$ 91