Summary of Significant Accounting Policies - Income and Other Taxes (Narrative) (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2018	Dec. 31, 2017
Southern Power
Tax Credit Carryforward [Line Items]
Reduction in tax basis of assets (percent)			50.00%
Southern Company Gas
Tax Credit Carryforward [Line Items]
Excise taxes collected	$ 32		$ 114	$ 100
Included In Operating Expenses | Southern Company Gas
Tax Credit Carryforward [Line Items]
Excise taxes collected	$ 31		$ 111	$ 98
Predecessor | Southern Company Gas
Tax Credit Carryforward [Line Items]
Excise taxes collected		$ 57
Predecessor | Included In Operating Expenses | Southern Company Gas
Tax Credit Carryforward [Line Items]
Excise taxes collected		$ 56