Income Taxes - Amortization of Deferred Credits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Line Items]
Amortization of deferred investment tax credits	$ 87	$ 79	$ 59
Southern Power
Income Tax Disclosure [Line Items]
Amortization of deferred investment tax credits	$ 58	$ 57	$ 37