Acquisitions and Dispositions - Southern Company's Sale of Gulf Power (Details) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
	Jan. 01, 2019	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2019
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Preliminary gain		$ 2,503	$ (1)
Disposal Group, Disposed of by Sale, Not Discontinued Operations | GULF POWER CO
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Proceeds from divestiture of businesses, net of cash divested	$ 5,800
Amount of indebtedness assumed at closing	$ 1,300
Disposal Group, Disposed of by Sale, Not Discontinued Operations | GULF POWER CO | Scenario, Forecast
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Preliminary gain				$ 2,500
Preliminary gain, net of tax				$ 1,300