Acquisitions and Dispositions - SO Acquisitions and Dispositions Narrative (Details) - USD ($) $ / shares in Units, $ in Millions			1 Months Ended	3 Months Ended								6 Months Ended	12 Months Ended
	Jan. 01, 2019	Jul. 01, 2016	May 31, 2016	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Total operating revenues													$ 23,495	$ 23,031	$ 19,896
Net income (loss) after dividends on preferred and preference stock				$ 269	$ 1,222	$ (127)	$ 936	$ 500	$ 1,109	$ (1,348)	$ 665		2,226	842	$ 2,448
PowerSecure International, Inc.
Business Acquisition [Line Items]
Business acquisition share price (in dollars per share)			$ 18.75
Business combination, consideration transferred, equity interests issued and issuable			$ 429
Southern Company Gas
Business Acquisition [Line Items]
Payments to acquire businesses		$ 8,000
Southern Company Gas
Business Acquisition [Line Items]
Total operating revenues												$ 1,652	3,909	3,920
Net income (loss) after dividends on preferred and preference stock				$ 78	$ 46	$ (31)	$ 279	$ (60)	$ 15	$ 49	$ 239	$ 114	$ 372	$ 243
Southern Company Gas | Southern Company
Business Acquisition [Line Items]
Business acquisition share price (in dollars per share)		$ 66
Subsequent Event | Disposal Group, Disposed of by Sale, Not Discontinued Operations | Gulf Power
Business Acquisition [Line Items]
Cash proceeds from sale of business	$ 5,800
Indebtedness assumed	$ 1,300