Summary of Significant Accounting Policies - Goodwill and Other Intangible Assets and Liabilities (Narrative) (Details) - USD ($)	3 Months Ended		6 Months Ended	12 Months Ended
	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Impairment charge	$ 0			$ 42,000,000
Intangible liabilities included in other deferred credits and liabilities	465,000,000			465,000,000	$ 691,000,000
Southern Company Gas
Finite-Lived Intangible Assets [Line Items]
Impairment charge	0	$ 42,000,000	$ 0	42,000,000	0
Intangible liabilities included in other deferred credits and liabilities	105,000,000			105,000,000	$ 118,000,000
Accumulated amortization of intangible liabilities	74,000,000			74,000,000
Remaining amortization to be recorded in 2019	17,000,000			17,000,000
Intangible liabilities | Southern Company Gas
Finite-Lived Intangible Assets [Line Items]
Intangible liabilities included in other deferred credits and liabilities	$ 91,000,000			$ 91,000,000